String api

Modified a bit of the API for Strings/Buffers as a reaction to the `appendgate` that occured earlier this week.

Now, copy methods (+,*,to_upper/lower) are deferred to String and in-place equivalents are available for Buffer.
en general, les fonction `to_*` retourne une copie (ou un truc immutable qui pretege l'original).

p'tet just `upper`? (meme chose pour `to_lower`)