Add Optional Support For Multiple References to an Object

# What This Does
Some state trees may need to reference an object more than once (such as the tree for my [fomod](https://www.npmjs.com/package/fomod) library). In essence, we store existing drafts when an off-by-default Immer class configuration option is enabled. This should be a painless solution. Specifics are described below.

## Implementation Details

* Two `WeakMap` are used to keep track of draft states and related data at different parts of the immerification process:
  * `existingStateMap_` maps a given base object to the first draft state created for it. This state includes a reference to the revokable draft.
    * If a state is referenced multiple times, it will be given a new `revoke_()` function that, once called the first time, calls the old `revoke_()` function. The result is that the final `revoke_()` must be called once for every requested draft before the Proxy is finally revoked. Since a proxy which has has its `revoke_()` method called should be considered revoked by all code paths, duplicate calls should not be an issue.
  * During finalization, `encounteredObjects` keeps track of objects we've finalized and doesn't traverse an object if it's already seen it. It prevents infinite recursion when circular references are present.
* Introduced the `extraParents_` property to the `ImmerBaseState` interface. This keeps track of additional values that would normally be attached to `parent_` so that functionality such as marking the parent state as modified is retained for objects with multiple parent objects
* For Maps and Sets, a proxy is established between the state and DraftMap/DraftSet classes to handle multiple references to these native classes while preserving the idea of having one DraftSet per reference.
* For Sets, each child draft has a single symbol value set so that a copy is prepared. (discussion needed; see TODOs below)
* During finalization, objects may have drafted children and, thus, even unmodified children are finalized in multi-ref mode
* To enable the feature, it is the same as other Immer class configuration options (such as `useStrictShallowCopy`). That is, either specify it in the config object passed to the class's constructor OR call the relevant method, `setAllowMultiRefs()`

> [!NOTE]
> Because of the extra computation involved with checking every proxied object against a map and traversing every object in a tree, enabling multi-ref will have a significant performance impact—even on trees which contain no repeated references.

# Tests
The file `__tests__/multiref.ts` contains a number of tests related to this multi-reference support implementation. Such tests seek to verify that:
* Direct circular references (which Immer tests for normally) do not throw an error when multi-ref is enabled
* When the properties of multiple references are modified, all references are modified
* Unmodified references to the same object are kept
* The same copy is provided for every reference (new references are strictly equivalent [`===`] just as the references before `produce()` would have been)

Tests are performed on all relevant object archetypes where applicable.

# Outstanding Discussion TODOs
* [ ] What to do regarding documentation
* [ ] Possible alternate solution for preparing copies for multi-reference DraftSet children
* [ ] Add an error for when WeakMap isn't supported in the current environment? (supported in every noteworthy browser and server environment since late 2015)

__tests__/__prod_snapshots__/manual.js.snap

@@ -2,6 +2,8 @@
 
 exports[`manual - proxy cannot finishDraft twice 1`] = `"Cannot perform 'get' on a proxy that has been revoked"`;
 
+exports[`manual - proxy cannot modify after finish 1`] = `"Cannot perform 'set' on a proxy that has been revoked"`;
+
 exports[`manual - proxy should check arguments 1`] = `"[Immer] minified error nr: 8. Full error at: https://bit.ly/3cXEKWf"`;
 
 exports[`manual - proxy should check arguments 2`] = `"[Immer] minified error nr: 8. Full error at: https://bit.ly/3cXEKWf"`;

__tests__/multiref.ts

@@ -0,0 +1,283 @@
+import {Immer, enableMapSet} from "../src/immer"
+import {inspect} from "util"
+
+// Implementation note: TypeScript says ES5 doesn't support iterating directly over a Set so I've used Array.from().
+// If the project is moved to a later JS feature set, we can drop the Array.from() and do `for (const value of ref)` instead.
+
+test("modified circular object", () => {
+	const immer = new Immer({allowMultiRefs: true})
+	const base = {a: 1, b: null} as any
+	base.b = base
+
+	const envs = ["production", "development", "testing"]
+	for (const env of envs) {
+		process.env.NODE_ENV = env
+		expect(() => {
+			const next = immer.produce(base, (draft: any) => {
+				draft.a = 2
+			})
+			expect(next).toEqual({a: 2, b: next})
+		}).not.toThrow()
+	}
+})
+
+test("unmodified circular object", () => {
+	const immer = new Immer({allowMultiRefs: true})
+	const base = {a: 1, b: null} as any
+	base.b = base
+
+	const envs = ["production", "development", "testing"]
+	for (const env of envs) {
+		process.env.NODE_ENV = env
+		expect(() => {
+			const next = immer.produce({state: null}, (draft: any) => {
+				draft.state = base
+			})
+			expect(next.state).toBe(base)
+		}).not.toThrow()
+	}
+})
+
+describe("access value & change child's child value", () => {
+	describe("with object", () => {
+		const immer = new Immer({allowMultiRefs: true})
+		const sameRef = {someNumber: 1, someString: "one"}
+		const objectOfRefs = {a: sameRef, b: sameRef, c: sameRef, d: sameRef}
+
+		const base = {
+			objectRef1: objectOfRefs,
+			objectRef2: objectOfRefs,
+			objectRef3: objectOfRefs,
+			objectRef4: objectOfRefs
+		}
+		const next = immer.produce(base, draft => {
+			draft.objectRef2.b.someNumber = 2
+			draft.objectRef3.c.someString = "two"
+		})
+
+		it("should have kept the Object refs the same", () => {
+			expect(next.objectRef1).toBe(next.objectRef2),
+				expect(next.objectRef2).toBe(next.objectRef3),
+				expect(next.objectRef3).toBe(next.objectRef4)
+		})
+
+		it("should have updated the values across everything", () => {
+			function verifyObjectReference(
+				ref: {[key: string]: {someNumber: number; someString: string}},
+				objectNum: number
+			) {
+				verifySingleReference(ref.a, objectNum, "a")
+				verifySingleReference(ref.b, objectNum, "b")
+				verifySingleReference(ref.c, objectNum, "c")
+				verifySingleReference(ref.d, objectNum, "d")
+			}
+
+			function verifySingleReference(
+				ref: {someNumber: number; someString: string},
+				objectNum: number,
+				refKey: string
+			) {
+				//it(`should have updated the values across everything (ref ${refKey.toUpperCase()} in object #${objectNum})`, () => {
+					expect(ref.someNumber).toBe(2)
+					expect(ref.someString).toBe("two")
+				//})
+			}
+
+			verifyObjectReference(next.objectRef1, 1)
+			verifyObjectReference(next.objectRef2, 2)
+			verifyObjectReference(next.objectRef3, 3)
+			verifyObjectReference(next.objectRef4, 4)
+		});
+	})
+
+	describe("with map", () => {
+		const immer = new Immer({allowMultiRefs: true})
+		enableMapSet()
+		const sameRef = {someNumber: 1, someString: "one"}
+		const mapOfRefs = new Map([
+			["a", sameRef],
+			["b", sameRef],
+			["c", sameRef],
+			["d", sameRef]
+		])
+
+		const base = {
+			mapRef1: mapOfRefs,
+			mapRef2: mapOfRefs,
+			mapRef3: mapOfRefs,
+			mapRef4: mapOfRefs
+		}
+		const next = immer.produce(base, draft => {
+			draft.mapRef2.get("b")!.someNumber = 2
+			draft.mapRef3.get("c")!.someString = "two"
+		})
+
+		it("should have kept the Map refs the same", () => {
+			expect(next.mapRef1).toBe(next.mapRef2),
+				expect(next.mapRef2).toBe(next.mapRef3),
+				expect(next.mapRef3).toBe(next.mapRef4)
+		})
+
+		it("should have updated the values across everything", () => {
+			function verifyMapReference(
+				ref: Map<string, {someNumber: number; someString: string}>,
+				mapNum: number
+			) {
+				verifySingleReference(ref.get("a")!, mapNum, "a")
+				verifySingleReference(ref.get("b")!, mapNum, "b")
+				verifySingleReference(ref.get("c")!, mapNum, "c")
+				verifySingleReference(ref.get("d")!, mapNum, "d")
+
+				//it(`should have the same child refs (map #${mapNum})`, () => {
+					expect(ref.get("a")).toBe(ref.get("b")),
+						expect(ref.get("b")).toBe(ref.get("c")),
+						expect(ref.get("c")).toBe(ref.get("d"))
+				//})
+			}
+
+			function verifySingleReference(
+				ref: {someNumber: number; someString: string},
+				mapNum: number,
+				refKey: string
+			) {
+				//it(`should have updated the values across everything (ref ${refKey.toUpperCase()} in map #${mapNum})`, () => {
+					expect(ref.someNumber).toBe(2)
+					expect(ref.someString).toBe("two")
+				//})
+			}
+
+			verifyMapReference(next.mapRef1, 1)
+			verifyMapReference(next.mapRef2, 2)
+			verifyMapReference(next.mapRef3, 3)
+			verifyMapReference(next.mapRef4, 4)
+
+		});
+	})
+
+	describe("with array", () => {
+		const immer = new Immer({allowMultiRefs: true})
+		const sameRef = {someNumber: 1, someString: "one"}
+		const arrayOfRefs = [sameRef, sameRef, sameRef, sameRef]
+
+		const base = {
+			arrayRef1: arrayOfRefs,
+			arrayRef2: arrayOfRefs,
+			arrayRef3: arrayOfRefs,
+			arrayRef4: arrayOfRefs
+		}
+		const next = immer.produce(base, draft => {
+			draft.arrayRef2[1].someNumber = 2
+			draft.arrayRef3[2].someString = "two"
+		})
+
+		it("should have kept the Array refs the same", () => {
+			expect(next.arrayRef1).toBe(next.arrayRef2),
+				expect(next.arrayRef2).toBe(next.arrayRef3),
+				expect(next.arrayRef3).toBe(next.arrayRef4)
+		})
+
+		it("should have updated the values across everything", () => {
+			function verifyArrayReference(
+				ref: {someNumber: number; someString: string}[],
+				arrayNum: number
+			) {
+				let i = 0
+				for (const value of ref) {
+					//it(`should have updated the values across everything (ref #${i++} in array #${arrayNum})`, () => {
+						verifySingleReference(value)
+					//})
+				}
+			}
+
+			function verifySingleReference(ref: {
+				someNumber: number
+				someString: string
+			}) {
+				expect(ref.someNumber).toBe(2)
+				expect(ref.someString).toBe("two")
+			}
+
+			verifyArrayReference(next.arrayRef1, 1)
+			verifyArrayReference(next.arrayRef2, 2)
+			verifyArrayReference(next.arrayRef3, 3)
+			verifyArrayReference(next.arrayRef4, 4)
+		});
+	})
+
+	describe("with set", () => {
+		const immer = new Immer({allowMultiRefs: true})
+		enableMapSet()
+		const sameRef = {someNumber: 1, someString: "one"}
+		const setOfRefs = new Set([{sameRef}, {sameRef}, {sameRef}, {sameRef}])
+
+		const base = {
+			setRef1: setOfRefs,
+			setRef2: setOfRefs,
+			setRef3: setOfRefs,
+			setRef4: setOfRefs
+		}
+		//console.log("base", inspect(base, {depth: 6, colors: true, compact: true}))
+
+		const next = immer.produce(base, draft => {
+			const set2Values = draft.setRef2.values()
+			set2Values.next()
+			set2Values.next().value.sameRef.someNumber = 2
+
+			const set3Values = draft.setRef3.values()
+			set3Values.next()
+			set3Values.next()
+			set3Values.next().value.sameRef.someString = "two"
+		})
+
+		//console.log(
+		//	"next",
+		//	inspect(next, {
+		//		depth: 20,
+		//		colors: true,
+		//		compact: true,
+		//		breakLength: Infinity
+		//	})
+		//)
+
+		it("should have kept the Set refs the same", () => {
+			expect(next.setRef1).toBe(next.setRef2),
+				expect(next.setRef2).toBe(next.setRef3),
+				expect(next.setRef3).toBe(next.setRef4)
+		})
+
+		it("should have updated the values across everything", () => {
+			function verifySetReference(
+				ref: Set<{sameRef: {someNumber: number; someString: string}}>,
+				setLetter: string
+			) {
+				//it(`should have the same child refs (set ${setLetter.toUpperCase()})`, () => {
+					let first = ref.values().next().value.sameRef
+					for (const value of Array.from(ref)) {
+						expect(value.sameRef).toBe(first)
+					}
+				//})
+
+				let i = 0
+				for (const value of Array.from(ref)) {
+					//it(`should have updated the values across everything (ref #${i++} in set ${setLetter.toUpperCase()})`, () => {
+						verifySingleReference(value.sameRef)
+					//})
+				}
+			}
+
+			function verifySingleReference(ref: {
+				someNumber: number
+				someString: string
+			}) {
+				expect(ref.someNumber).toBe(2)
+				expect(ref.someString).toBe("two")
+			}
+
+			verifySetReference(next.setRef1, "a")
+			verifySetReference(next.setRef2, "b")
+			verifySetReference(next.setRef3, "c")
+			verifySetReference(next.setRef4, "d")
+
+		});
+	})
+})