WebIDL binder: Use WeakRefs in cache

[kripken]

The cache maps C pointers (integers) to JS object wrappers. It
currently keeps objects alive forever. This is normally ok as the
objects should be manually destroy()ed, however, we do allow
wrapping a class as a parent class, and then it ends up in
multiple caches, see kripken/ammo.js#284

Using WeakRefs where available could avoid this. However, until
they are present everywhere, perhaps we could analyze the
inheritance graph and clean up parent caches too, but that may
be tricky especially with multiple/virtual/etc. inheritance.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 30 days. Feel free to re-open at any time if this issue is still relevant.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 30 days. Feel free to re-open at any time if this issue is still relevant.