Rename loop labels.

[iago-lito]

#6907 made me realize that we cannot rename loop labels yet, like we rename other things:

fn main() {
    'a: loop { // Cannot rename 'a: No references found at position.
        for i in 0..5 {
            if i > 1 {
                break 'a;
            }
        }
    }
}

I love this loop-labelling feature, but I admit it's rare enough that hand-refactoring is ok and that adding support for loop label reference search can remain low-priority.. but it's worth noticing :)