Merge: String block optimisations

This stack of commits is a batch of brand-new optimisations on `String`, especially regarding the manipulation of UTF-8 characters.

As a baseline, last Tuesday @privat tried the JSON parser on a 30MiB json file, the execution took approximately 10 seconds for ~38 GIr with Valgrind, now and with his Nitcc upgrades, the total runtime is less than 2 seconds for ~11 GIr with Valgrind.

Pull-Request: #1885
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>