Removed rev_iter in favor of DoubleEndedIterator

In the process, `Splits` got changed to be more like `CharSplits` in `str` to present the DEI interface.

Note that `treemap` still has a `rev_iter` function because it seems like it would be a significant interface change to expose a DEI - the iterator would have to gain an extra pointer, the completion checks would be more complicated, and it isn't easy to check that such an implementation is correct due to the use of unsafety to subvert the aliasing properties of `&mut`.

This fixes #9391.