Merge: Utf8 strings - noindex

Added a few features missing from UTF8 Strings without index.

NOTE : It is slow and space-consuming (lots of allocated characters and no GC), however by experimenting a bit (read : force UnicodeChar as a Universal intern class), the performance is surprisingly close to the old ones and the memory-use becomes acceptable once again :
* < 10% loss for iteration (with iterator, not index, this is much slower) and concat
* substring is slower by a significant margin (75%), however considering the cost of the ancient one (close to none), I guess this is still acceptable.

Pull-Request: #684
Reviewed-by: Jean Privat <jean@pryen.org>