Add support to compile for Swift 4.0 (#1)

A few Overflow Calculating methods are changed in Swift-4.0, this PR takes care of that.

S2GeometrySwift/Classes/S2CellId.swift

@@ -214,12 +214,12 @@ public struct S2CellId: Comparable, Hashable {
 	public var level: Int {
 		// Fast path for leaf cells.
 		guard !isLeaf else { return S2CellId.maxLevel }
-		var x = Int32(truncatingBitPattern: id)
+        var x = Int32(truncatingIfNeeded: id)
 		var level = -1
 		if x != 0 {
 			level += 16
 		} else {
-			x = Int32(truncatingBitPattern: id >> Int64(32))
+			x = Int32(truncatingIfNeeded: id >> Int64(32))
 		}
 		// We only need to look at even-numbered bits to determine the
 		// level of a valid cell id.
@@ -355,8 +355,7 @@ public struct S2CellId: Comparable, Hashable {
 	public func next() -> S2CellId {
 
 
-
-		return S2CellId(id: Int64.addWithOverflow(id, lowestOnBit << 1).0)
+		return S2CellId(id: id.addingReportingOverflow(lowestOnBit << 1).partialValue)
 	}
 
 	/**
@@ -365,7 +364,7 @@ public struct S2CellId: Comparable, Hashable {
 		around from the last face to the first or vice versa.
 	*/
 	public func prev() -> S2CellId {
-		return S2CellId(id: Int64.subtractWithOverflow(id, lowestOnBit << 1).0)
+        return S2CellId(id: id.subtractingReportingOverflow(lowestOnBit << 1).partialValue)
 	}
 
 	/**
@@ -772,7 +771,7 @@ public struct S2CellId: Comparable, Hashable {
 
 //		UInt64
 
-		return Int(truncatingBitPattern: (uid >> 32) + uid)
+        return Int(truncatingIfNeeded: (uid >> 32) + uid)
 	}
 
 	/**

S2GeometrySwift/Classes/S2Point.swift

@@ -98,9 +98,11 @@ public struct S2Point: Comparable, Hashable {
 
 	public var hashValue: Int {
 		var value: UInt64 = 17
-		value = UInt64.addWithOverflow(value, UInt64.multiplyWithOverflow(37, UInt64.addWithOverflow(value, abs(x).bitPattern).0).0).0
-		value = UInt64.addWithOverflow(value, UInt64.multiplyWithOverflow(37, UInt64.addWithOverflow(value, abs(y).bitPattern).0).0).0
-		value = UInt64.addWithOverflow(value, UInt64.multiplyWithOverflow(37, UInt64.addWithOverflow(value, abs(z).bitPattern).0).0).0
+        value = value.addingReportingOverflow(UInt64(37).multipliedReportingOverflow(by: value.addingReportingOverflow(abs(x).bitPattern).partialValue).partialValue).partialValue
+
+        value = value.addingReportingOverflow(UInt64(37).multipliedReportingOverflow(by: value.addingReportingOverflow(abs(x).bitPattern).partialValue).partialValue).partialValue
+
+        value = value.addingReportingOverflow(UInt64(37).multipliedReportingOverflow(by: value.addingReportingOverflow(abs(x).bitPattern).partialValue).partialValue).partialValue
 		value ^= (value >> 32)
         return Int(Int64(bitPattern: value))
 	}