Use correct case folding for case insensitive match

[Geal]

the current code of Compare::compare_no_case for &str does a to_lowercase on the input and the tag to compare to. This does not take into account complex cases when doing an uppercase or lowercase on an UTF-8 string.

The caseless crate provides methods to do caseless comparisons.

Things to check for:

• if the input is not large enough, we do a partial match on a prefix. But the prefix should not be obtained before matching since the length before and after case folding
• once the match is done, how to convert the size to before case folding, since a sub slice must be calculated
• put that version behind a compiler flag?