Merge: UTF-8 Strings without index

Another WIP version of UTF-8 Strings, without indexing this time.

This is more likely to become the final implementation, depending on the performance of it. I'll add benches for the versions of UTF-8 to examine their impact and decide a bit later.

This implementation being less rigid than the other, more of the API is supported, still, more work needs to be done.

Pull-Request: #655
Reviewed-by: Jean Privat <jean@pryen.org>