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 would be okay with dropping everything but `NoEncryption` and `BestAvailable` if we think that the API `dump_pem(serializer)` gives us enough flexibility to handle the more complex options in the future.