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.
Logiquement la page de la propriété devrait déjà linker au module non?

Mais pour clarifier la chose, je songe à déplacer la documentation principale sur la classe Regex.