Made everything work with V8 version 3.28.71.

In the process, switched away from trying to make weak references to
registered functions. This was a pain in the ass when tracking API
changes, and necessitated all sorts of hacks to encourage garbage
collection in testing. These hacks weren't principled -- finalizers
still aren't guaranteed to run; in particular they seem to not run when
disposing an isolate. So don't try to pretend that we can avoid leaks
in the embedding program with this mechanism.
Removed some further uses of Isolate::GetCurrent.