Merge: UTF-8 Regex

This PR closes #1684

Instead of making `byte_to_char_index` public, it has been removed as it had no real reason to live.

Names are corrected and should correctly reflect their use.
Some examples of regular expressions with UTF-8 have been included.

Note however that the C-library underneath does not have UTF-8 semantics, as such, when using repetition operators on UTF-8 strings, capture the problematic characters with parentheses as in the example, or else the result will be erroneous.

Additionally, performances should be a bit better since less allocations and copy_to should be done.

Pull-Request: #1692
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>