Consider adding `#[must_use]` in `Colorize` methods

[synek317]

It makes a lot of sense to add #[must_use] for methods in the Colorize trait.

I discovered It funnily by writing this code:

fn foo(s: &mut str) {
  // ...
  s.clear();
}

Of course, I should have used &mut String for my mutable buffer. But this compiled just fine since it calls Colorized::clear. With #[must_use], it would be easier to spot.

Even without this example, I think #[must_use] is a viable addition.