Fixes #45: WeakDictionary is missing.

This commit implements WeakDictionary. As was agreed
months ago with P. Van Roy, this implementation has
post-mortem semantics, whereas Mozart 1 had pre-mortem
semantics for WeakDictionaries.

This means that instead of sending Key#Value to the
stream when Value is about to become dead, we send
Key#unit to the stream when Value is dead.