This is in preparation of implementing %IteratorHelperPrototype%.return.
That will invoke GeneratorResumeAbrupt, which will execute the generator
with an abrupt completion. At that time, we must take care to close the
current inner iterator.
This is required for %IteratorHelperPrototype%.return, which needs to
operate on the internal slots of the base GeneratorObject. Doing so also
provides us with the appropriate VM to be used in invocations to the
overridden IteratorHelper::execute.
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.
