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
Yeah, I was curious if one could build an encrypted PKCS1-style PEM with a DEK-Info. Obviously it's utterly pointless, but that doesn't mean it hasn't been done. If there aren't even crazy people using it though (and I've certainly never seen one) then I'm happy to say we should just drop password from this interface entirely.