Rename WeakFactory to FinalizationGroup

[tschneidereit]

I think we should rename WeakFactory, for a couple of reasons:

1. Grouping lifetimes is the function of this class that's more immediately relevant to developers
2. It's function as a mechanism for removing the ability to create WeakRefs (while keeping the ability to hand them out) is a very advanced concept that we don't need to put focus on

Realistically speaking, most developers won't have strong intuitions about any of the terms in this domain. Nevertheless, I think we should try to use names that at least guide them in the right direction. In particular, the concept of grouping lifetimes of a set of WeakRefs is important, and I think the solution we settled on is excellent. Now we also need to make it as easy as possible for developers to understand this solution :)

I'm not saying that FinalizationGroup is a fantastic name—in particular it emphasizes only one half of what is provided—, but I do think that it at least points in the right direction, providing a good starting point for learning about the concepts involved via sources like MDN.

@erights, @dtribble, WDYT?

[littledan]

Thanks for the suggestion; this name change seems to be complete.