Merge: UTF-8 Strings

Merge: UTF-8 Strings

In regard to #1262, and after many rewritings, here's a fully-functional prototype for UTF-8 compliant Strings !

Many things have changed and now, operations on String will need to be considered differently to avoid bad surprises, namely:

- FlatBuffers are slow as hell when getting chars or modifying in-place (and an in-place modification can degenerate if a single-byte char is modified by a multibyte char)
- Length is now potentially expensive (especially within FlatBuffer where it is not cached)
- Indexed access is now O(n), though it is cached for local accesses (except in FlatBuffer)

Performances, for the user time of `nitc src/nitc.nit -o bin/nitc`, has gone from 4.55s to 4.80s, so +5.5%; we have seen worse.

So a little slowdown, but definitely acceptable when introducing UTF-8 in Strings.

Pull-Request: #1277
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>