Merge: Faster hex parsing

As said in #1895, we need faster parsing of UTF-16 escaping sequences, this PR is the answer.

It makes the runtime of the `large_escaped` benchmark go down from ~5s to ~3.5s, and with valgrind, from 26GIr to 20GIr

Note: based on #1886, only the 4 last commits are of interest here

Pull-Request: #1896
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/core/text/abstract_text.nit
Simple merge