Look into ways to cross-verify implementation

### What is wrong?

https://github.com/ethereum/py_ecc/issues/24#issuecomment-445506905

This is a little frightening (no blame intended). It would be great if we could come up with a way to cross check this implementation with a known good one or at least to have some additional static test vectors we could test against.

### How can it be fixed

Look into what other implementations exist and how we could use them in a CI environment or manually extract some test vectors from them.