New ES proposal: Set and Map `of` and `from` methods

[leobalter]

https://github.com/leobalter/proposal-setmap-offrom

After catching a conversation on twitter I volunteered to work on this proposal, which adds the of and from methods to Set, Map, WeakSet and WeakMap constructors.

It got my personal interest as I wrote tests (on test262) for Array.of, Array.from, the same respective tests for TypedArrays and tests for Map, WeakSet and WeakMap. This was some nice opportunity to continue API work with some very useful features.

[leobalter]

It advanced to stage 1, but I still got some homework to do. Updates will come soon