std::str: bench tests for .split_iter()
std::str: Tune CharSplitIterator after benchmarks

Implement Huon Wilson's suggestions (since the benchmarks agree!).

Use `self.sep.matches(byte as char) && byte < 128u8` to match in the
only_ascii case so that mistaken matches outside the ascii range can't
create invalid substrings.

Put the conditional on only_ascii outside the loop.