Merge: escape_to_c: Escape trigraphs

Produce more standard-compliant code instead of relying on the default behaviour of gcc (or on its `-Wno-trigraphs` flag).

See also: https://github.com/nitlang/nit/pull/1949/commits/000ac8de3c64790b0fbb0e868e406a6a280f3fa9

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>

Pull-Request: #2151
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

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