:extend mixins

This feature request was proposed as a solution to this issue https://github.com/cloudhead/less.js/issues/1155

This is the synopsis

The awesome thing about mixins is that they speed up development, keep my work surface clean, I only have to write them once, and I know what the compiled result will be. However, a disadvantage of mixins is that they aren't DRY. Especially when you use "utility" mixins a lot, like clearfix. This is where extends could be a much better option. In fact, this is a great, concrete use case for extending mixins. It would basically work like extending nested selectors, accept with mixins. So mixins wouldn't change at all, they would still work the same way. If you have a mixin and don't use it, it won't show in the compiled CSS. If you do use it, it's properties will still show up in the compiled code in each selector where it was used. And if you extend a mixin, the selector (or selectors) extending the mixin will show up in place of the mixin. So if you do this:

.clearfix() {
    // stuff
}
.navbar {
    &:extend(.clearfix());
}
.banner {
    &:extend(.clearfix());
}

and the compiled result will be:

.navbar,
.banner {
   // clearfix stuff
}

So the mixins properties are still inherited by the selectors that extended it, but the mixin (selector) itself doesn't show up in the compiled result.

This would make both extends and mixins far more powerful than they are on their own.

Start by reviewing the linked issue #1155 and the requested :extend mixin examples. Define the intended compilation semantics and add coverage for the shown .clearfix(), .navbar, and .banner case; done means the generated CSS matches the requested output and existing mixin and extend behavior remains compatible.