This prototype is a bit tricky in that we need to maintain the iteration
state of the mapped iterator's inner iterator as we return values to the
caller. To do this, we create a FlatMapIterator helper to perform the
steps that apply to the current iteration state.
This uses a new Iterator type called IteratorHelper. This does not
implement IteratorHelper.prototype.return as that relies on generator
objects (i.e. the internal slots of JS::GeneratorObject), which are not
hooked up here.
Iterator.from creates an Iterator from either an existing iterator or
an iterator-like object. In the latter case, it sets the prototype of
the returned iterator to WrapForValidIteratorPrototype to wrap around
the iterator-like object's iteration methods.
