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 
Fair enough, but in practice I've never seen that used by OpenSSL or any other libraries other than in the PKCS#1 case where there was no other option. This is why for PKCS#1 the only encrypted keys are PEM keys, and you can't create an encrypted DER key (whereas you can with PKCS#8).