Using NSHashTable as set of weak objects

[almassapargali]

I think https://github.com/objc-zen/objc-zen-book#multiple-delegation part of article should mention NSHashTable as a way to store weak references without creating ZOCWeakObject class.

[luca-bernardi]

For sure NSHashTable is one way todo that, another way is NSPointerArray (that is the one that I'm using using in my library https://github.com/lukabernardi/LBDelegateMatrioska).

I think we can expand the section to give more options. Feel free, If you want, to send a pull request with those improvements.