Remove remainder field from MutChunkIter

This field is no longer necessary now that #9629 is fixed since we can just
access the length of the remaining slice directly.
Update next() and size_hint() for MutSpliterIterator

Update the next() method to just return self.v in the case that we've reached
the last element that the iterator will yield. This produces equivalent
behavior as before, but without the cost of updating the field.

Update the size_hint() method to return a usable hint now that #9629 is fixed.
Implement size_hint() for ByRef iterator