[Typed objects] Implement weak references

[trev]

Background

#258 implemented reference counting for garbage collection which means that reference cycles will cause memory leaks.

What to change

Implement a weak reference mechanism to be used to avoid reference cycles.

[fhd]

Since the Emscripten based approach is looking rather good, I'm not sure we really want to continue to work on Typed Objects. Makes it a P5.

[trev]

In fact, at that point I am certain that we are dropping typed objects and go with Emscripten instead. The performance issues we had with Emscripten initially are bound to affect us with typed objects as well (important points: static generation of JS bindings, stack-based allocations, likely more). Fixing these issues was already non-trivial with Emscripten, but with our own toolchain it will require far more effort - this isn't going to pay off if we have a reasonable and maintained alternative.