Add regex_if! and regex_switch!

[Canop]

Those two macros allow conditional computation based on regex matching, with variables declared from the named capturing groups.

Of course the regexes are lazy static and checked at compile time.

Example with regex_if!

 let s = "gray(15)";
 let gray = regex_if!(
        r#"^gr(a|e)y\((?<level>\d{1,2})\)$"#,
        s,
        level.parse()?,
 );
 assert_eq!(gray, Some(15));

Example with regex_switch!

#[derive(Debug, PartialEq)]
enum Color {
    Grey(u8),
    Pink,
    Rgb(u8, u8, u8),
}
let input = "rgb(1, 2, 3)";
let color = regex_switch!(input,
    r#"^gr(a|e)y\((?<level>\d{1,2})\)$"#i => {
        Color::Grey(level.parse()?)
    }
    "^pink"i => Color::Pink,
    r#"^rgb\((?<r>\d+),\s*(?<g>\d+),\s*(?<b>\d+),?\)$"#i => Color::Rgb (
        r.parse()?,
        g.parse()?,
        b.parse()?,
    ),
);
assert_eq!(color, Some(Color::Rgb(1, 2, 3)));

[Canop]

I might add the bytes_ variant. I'd like some feedback before I merge this.