WIP: Support nested methods and event emitters

[gmaclennan]

This allows you to call a nested method on the server e.g.
client.myMethod().otherMethod()

This is only a proof-of-concept and will not yet work. Implementing this probably requires support for Weak References and Finalizers that were added in Node v14.6.0+. They are needed because the server needs to keep a reference to the object returned by client.myMethod() for as long as the client does. E.g.

const subClient = client.myMethod()
const result = await subClient.otherMethod()

We need a WeakRef to garbage collect subClient on the server when it is garbage collected on the client. Otherwise memory will grow on the server since we need to keep around any returned values forever.

It might be too difficult to implement this bug-free, it's probably better to avoid this pattern for an API that is going to be called over RPC.