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.
With AsyncSSH, I did something like this, with my export function for public keys taking only a "format" argument and my export function for private keys taking that and passphrase, cipher, hash, and version arguments (all with reasonable defaults so you could specify only the passphrase to get reasonable strength encryption, or no passphrase to output the key unencrypted). Given that the encryption is largely orthogonal to the format of the output, this may not be a bad approach.

In my case, the "format" argument let you choose between PKCS#1 DER and PEM and PKCS#8 DER and PEM for private keys, and all of those plus OpenSSH and RFC4716 format for public keys. That's something else to keep in mind -- the choices for format will vary between private & public.