Make handles one-to-one, instead of merely unique

[DUOLabs333]

This fixes a bug where getcwd returns ENOENT.

[willscott]

I'm tempted to just add the invalidate to the interface rather than also the update? It looks like invalidation should be enough to implement the updating of handles if potentially slightly less efficient.

[DUOLabs333]

Yeah, it should be enough --- it's just that it's possible that a client (incorrectly) assumes that Rename does not invalidate handles, so we should be able to switch if needed.

[willscott]

if the client assumes the handle isn't invalidated it should get a stale handle error on it's next request and should then do an update, right?

We need to invalidate the old and new directories, is the main thing, right?

[DUOLabs333]

1. It should, yes (in theory).

2. Only the old filename.

[DUOLabs333]

I think you're right --- I'll remove the Update method after testing it (I didn't test this specific PR --- I based it off of a simpler POC that I did test).

[DUOLabs333]

Ok, tested the code, and removed the UpdateHandle method.