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.
For full generality, the option to serialize with encryption should take argument for which cipher to use, which hash to use, which version of PBE to use (1 or 2), and a passphrase. As you said, the KDF would also be implied here, but it really can't be selected directly (or at least not independently).

It would also be good if the passphrase could be either a 'bytes' or a 'str', so that depending on the KDF implied by the other choices the right conversion from Unicode to bytes is selected.