add support for tuples to {Async,}Iterator.prototype.flatMap and {Async,}Iterator.from

[michaelficarra]

Iterator helpers advanced to stage 3 today. As discussed in plenary, we would like the R&T proposal to add support for tuples to these 4 methods:

• Iterator.from
• AsyncIterator.from
• Iterator.prototype.flatMap
• AsyncIterator.prototype.flatMap

This should be pretty easy to do by just modifying GetIteratorFlattenable which is shared between all of them.