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.
We don't support passphrases on loading them, so I think we shouldn't on
serializing them,

On Fri Dec 26 2014 at 9:47:38 AM Alex Stapleton <notifications@github.com>
wrote:

> In docs/hazmat/primitives/asymmetric/serialization.rst
> <https://github.com/pyca/cryptography/pull/1503#discussion-diff-22286080>:
>
> > @@ -114,6 +114,53 @@ all begin with ``-----BEGIN {format}-----`` and end with ``-----END
> >          is not supported by the backend.
> >
> >
> > +Serializers
> > +~~~~~~~~~~~
> > +
> > +.. class:: PKCS8(encryption_algorithm)
> > +
> > +    .. versionadded:: 0.7
> > +
> > +    A serializer for the PKCS #8 format.
> > +
> > +    :param encryption_algorithm: An object conforming to the
> > +        :class:`~cryptography.hazmat.primitives.serialization.KeySerializationEncryption`
> > +        interface.
> > +
> > +.. class:: TraditionalOpenSSL(encryption_algorithm)
>
> IIRC you can encrypt public keys using the traditional OpenSSL format, I
> don't think anyone ever does though.
>
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> <https://github.com/pyca/cryptography/pull/1503/files#r22286080>.
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