Implement iterable support for the Set's ctor.

Notes

1. Is there a way to compare an ejsval to false/true, without having to decor it with EJSVAL_TO_BOOLEAN?
2. You had some internal functions in ejs-promises.c, which resemble those ones needed by the iterator stuff in the spec. I'm not quite sure if those are supposed to be the very same - renamed yours for now, adding a 'P' suffix.