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.
In terms of naming, I would lean toward using the name PKCS1 rather than TraditionalOpenSSL. The format is not particularly specific to OpenSSL, and PKCS1 is more consistent with the other main option here of PKCS8. We may actually need to expand this to names like PKCS1_DER, PKCS1_PEM, PKCS8_DER, and PKCS8_PEM, and there I think the shorter name is even more beneficial.