impl next_back() for CharSplitIterator, and methods .rsplit_iter(), .rsplitn_iter()

Make CharSplitIterator double-ended which is simple given that the operation is symmetric, once the split-N feature is factored out into its own adaptor.

`.rsplitn_iter()` allows splitting `N` times from the back of a string, so it is a completely new feature. With the double-ended impl, `.split_iter()`, `.line_iter()`, `.word_iter()` all allow picking off elements from either end.

`split_options_iter` is removed with the factoring of the split- and split-N- iterators, instead there is `split_terminator_iter`.