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 think it'd be clearer to write this as:

```py
if isinstance(s, PKCS8):
    # ...
elif isinstance(s, TraditionalOpenSSL):
    # ...
else:
   raise TypeError("...")