Implement IterableWeakSet IterableWeakMap classes

[bogeeee]

Needed to properly clean up callbacks on connection but not cause a memory leak by all the on-socketconnection-close handlers itsself: The ClientCallbacks / ClientCallbacksForEntites should be allowed to be forgotten. Also ClientCallbacksForEntities's keys

These classes can be like the ones in the not-so-weak package, but these have the flaws that they

• don't work, becaues they call registry.register with WeakRef objects as values and unregisterTokens. But both work only with normal objects and fail silently otherwise (=don't fire) - have tested this !
• Their type parameters are faulty and also their iterator's types don't match those of the base class - maintainer not willing to fix that.

So that cries for a reimplementation