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 
What do you mean by PEM encryption? Are you talking about the "BEGIN RSA PRIVATE KEY" format with the Proc-Type and DEK-Info headers? If so, that's specific to PKCS#1. For PCKS#8, it uses "BEGIN PRIVATE KEY" (without "RSA") or "BEGIN ENCRYPTED PRIVATE KEY" without any PEM headers, since the encryption is done as a wrapper at the ASN.1 DER level and is part of the BASE64-encoded PEM data.