UTF-8 Strings without index

Another WIP version of UTF-8 Strings, without indexing this time.

This is more likely to become the final implementation, depending on the performance of it. I'll add benches for the versions of UTF-8 to examine their impact and decide a bit later.

This implementation being less rigid than the other, more of the API is supported, still, more work needs to be done.
C'est juste un hack pour le < en fait, mais vu que c'est expérimental comme PR, y'a peut-être pas besoin ouais.
D'autant que si (quand?) les strings sont implémentées comme ça, il disparaîtra.