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.
Vu que c'est expérimental j'me le suis permis ouais, si (quand ?) ça passe en Standard, ça remplacera l'ancien itérateur sur chars.