Initial public key loading work

- [x] I think the use of PKCS8/PEM might be wrong?
- [x] Docs
- [x] Tests (need vectors, where do they come from? Just re-process the private keys?)
- [x] Still a bit too much code duplication
I think mostly just not getting around to it. OpenSSL is capable of handling serialised EC keys about as well as anything else IIRC.