Utf8 strings

(Re?)Introducing Unicode strings (with UTF-8) with mandatory indexes for now, the module is incomplete yet (don't go and try to concatenate two UTF-8 FlatStrings, you'll be surprised by the result).

There's still some work to do on the global API, but here's a WIP version that should not break anything, any comments are welcome !
Est-ce vraiment pertinent de faire le to_upper pour tout l'Unicode ? Il y a beaucoup de langues qui n'ont même pas ce concept.