the new WeakKeyDictionary

I've been running with the new WeakKeyDictionary lately.

    http://bugs.squeak.org/view.php?id=6348

It seems to speed things up, but unfortunately I'm still encountering
occasional problems; either "No free space" or "Index out of range"
errors.  The latter, particularly, when saving the image.

I haven't been able to make a simple script that reproduces the
problem, but here is at least a stack trace.

I do also have an image saved that, upon launching, immediately
encounters the problem after starting up.

Thanks for your work on this Martin, I hope we can get it completely
bullet-proof..

  - Chris