Initial public key loading work

- [ ] I think the use of PKCS8/PEM might be wrong?
- [ ] Docs
- [ ] Tests (need vectors, where do they come from? Just re-process the private keys?)
- [ ] Still a bit too much code duplication
Do you have an opinion about what we should call it then? As far as I can tell, the format itself is "base64 asn1 encoded public keys using the PKCS1 asn represetnation"