support String parameters in Iterator.from and AsyncIterator.from

[michaelficarra]

As agreed upon in committee today as a condition for stage 3 advancement. Fixes #244.

[zloirock]

It's strange to limit it only to strings - why not ToObject for everything? Upcoming proposals will have other iterable primitives / non-objects. For example, Typles. In some cases, it could theoretically require forced polyfilling of those .from methods.

[michaelficarra]

We plan to have R&T proposal add support for tuples in GetIteratorFlattenable. Other primitives are much more likely to be an error than intending to be iterated. If a caller really wants to iterate any value, they can accommodate that with a call to Object().

edit: See tc39/proposal-record-tuple#369.