Naively ported to Swift 2.0

[jpsim]

Biggest changes here are:

1. No longer needs to Box
2. Swapped protocol free-functions to the new stdlib's protocol extension versions
3. Printable -> CustomStringConvertible

[robrix]

Maybe we should remove the external label on the second parameter instead. I don’t have a strong opinion other than to note that usage is perhaps not a terribly helpful external label.

[jspahrsummers]

@jpsim Thanks for taking this on! ✨

I'd like master to match the latest production version of Swift, so I don't think we want to pull this PR in as-is.

However, since it will probably be useful to bring in these changes (for SourceKitten, at the very least), I think maintaining a branch with Swift 2 support would be 💎. To that end, I've added you as a collaborator—feel free to push your branch to the main repo. 🙇

[jpsim]

Makes total sense, I won't be merging Swift 2 support in my own repos until it goes GM either. I'll push up this branch soon. Thanks for the invite!