Implement String.empty?

[korbin]

This pull request implements String.empty?/1.

String.empty? allows for cleaner conditional blocks and DRYs string emptiness logic (without equality checks, etc.).

I really like how this composes!

[josevalim]

Thanks but we don't have List.empty?, nor Integer.zero? and so on. And most of the times is better to pattern match and providing things like empty? partially push us into the wrong direction.