Minor cleanups now that #9629 is fixed

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