Support for traditional OpenSSL and PKCS8 RSA private key serialization

This temporarily uses the backend to do it until the alternate approach is ready. DSA/ECDSA and public key support will follow if this is deemed worthy.
I don't really like having a method on keys to serialise them. If (as suggested elsewhere) it's just named `dump` it's just going to immediately hand control back to one of the instances passed to it. It also doesn't match how keys are loaded at all.