Merge: 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 !

Pull-Request: #602
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>