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.
Another solution would be to move the encryption option from the serializer
constructor to `dump_pem`, then we just don't have that argument on public
keys. It's a slightly wrong abstraction though. Thoughts?

On Sat Dec 27 2014 at 3:57:44 PM Ron Frederick <notifications@github.com>
wrote:

> In docs/hazmat/primitives/interfaces.rst
> <https://github.com/pyca/cryptography/pull/1503#discussion-diff-22295257>:
>
> > +    Extends :class:`RSAPrivateKey`. Deprecated in favor of
> > +    :class:`RSAPrivateKeyWithSerialization`.
> > +
> > +    .. method:: private_numbers()
> > +
> > +        Create a
> > +        :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateNumbers`
> > +        object.
> > +
> > +        :returns: An
> > +            :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPrivateNumbers`
> > +            instance.
> > +
> > +.. class:: RSAPrivateKeyWithSerialization
> > +
> > +    .. versionadded:: 0.7
>
> Seconded -- perhaps NoEncryption could be a default, though, and that
> could be the only option allowed in the public key case so it would be as
> if that argument didn't exist there.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/pyca/cryptography/pull/1503/files#r22295257>.
>