Regular expressions & prop to bring back a better `String::has(Pattern)`

As asked by everyone...

`String::has(Pattern)` is much faster than `String::index_of(Pattern)` with regex. It can be optimized even more with `optimize_is_in`.

I'll be waiting after the test to make sure that it doesn't break anything.

What do you think of the doc/tests on the protected methods? I don't have any info to add, only tests and `requires`.

The only TODO would be to implement subexpressions; to get the the substrings of the parenthetical subexpressions of a single match. It is not the most common use but could be useful in some compilers.
Il s'agit déjà de doc privée qui ne sera normalement pas généré.

L'accès à la couche native d'un wrapper par un intrude import est rendu assez standard. Mais en général ce n'est utiliser que pour implémenter d'autres bibliothèque sans que les gens aies à réécrire la couche de bas niveau.