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 better hint now that #9629 is fixed.
Implement size_hint() for ByRef iterator
auto merge of #10930 : DaGenix/rust/remove-unnecessary-fields, r=alexcrichton

3 minor clean-ups now that #9629 is fixed:

* Update MutChunkIter to remove the ```remainder``` that existed just to allow the size_hint() method to be implemented. This is no longer necessary since we can just access the length of the slice directly.
* Update MutSplitIterator to address the FIXME in its size_hint() method. This method was only partially implemented due to the issue. Also, implement a minor optimization in the case that its the last iteration.
* Update ByRef iterator to implement the size_hint() method.

I noticed that MutSplitIterator returns an empty slice if called on an empty slice. I don't know if this is intended or not, but I left the ```finished``` field in-place to preserve this behavior.

@TeXitoi @blake2-ppc