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.
Y'a une faille dans le parser markdown qui fait qu'il ne considère pas les `REQUIRE:` en minuscule.

Là encore c'est pas de la mauvaise fois, juste que je trouve ça plus élégant en majuscule. En plus ça ce remarque mieux dans la doc non coloriée.

Parcontre j'avoue que `doc_down` pourrait lever un warning.