SSH key support

[str4d]

Closes #6.

[str4d]

Hmm, this will be tricky to finish:

• The rsa crate has an open PR for OAEP (Add OAEP Encryption & Decryption RustCrypto/RSA#18) but it is not complete and needs rebasing.
• There aren't any crates that parse OpenSSH pubkeys and also cover all of the following:
  • Parse OpenSSH private keys.
  • Expose the pubkey for a private key (either extracted from the privkey format or by reserializing).
  • Expose the internals of the keys for transposing into the rsa crate.

[str4d]

Well, this was... unpleasant.