Simplify the hell out of things.

* No more `element`, just use `byte`.
* No more `polynomial`, just use `[]byte`.
* Inline `add` and `sub`.
* Remove `log`. Use the table instead.
* Ditch test helpers. Literally no memory of what I was trying to do there.
* Use `byte` and `map[byte][]byte` for the public API.
* Re-flow some comments.
Bump Travis-CI to 1.3.