Encode strings with CESU-8

Related to #11.

We need to figure out in which cases (invalid data?) this will raise errors.
> Ah, so it I was still using escaped hex it'd be fine? `"\xf0\x0f..."`?

Yes, in that case `force_encoding` would be correct.

> If we added `# -*- coding: utf-8 -*-` to this file, would that ensure the literal was always initialized as UTF-8?

Yes. The Ruby docs even recommend doing that for all source files:

> Ruby source files should declare its script encoding by a magic comment even when they only depend on US-ASCII strings or regular expressions.