[WIP] RSAPrivateKey support

- [ ] Documentation
- [ ] Support configurable ciphers for to_pkcs8
- [ ] Fix buffer sizing for pkcs8

There are also 7 test vectors added here. These are all a single 2048-bit private key (public exponent 65537) I generated and then converted into the following formats:

* PKCS1 PEM
* PKCS1 DER
* PKCS1 encrypted PEM
* PKCS8 PEM
* PKCS8 DER
* PKCS8 encrypted PEM
* PKCS8 encrypted DER

RSAPublicKey will be in another PR (trying to keep the size of this PR down).