Support AES keys / symmetric keys

[TheButlah]

I'd like to use this library for representing both ed25519 keys (supported) and AES content encryption keys (unsupported). From what I can tell, RFC7517 defines how to specify those keys.

Actually performing encryption or decryption should be out of scope, IMO. I just want a way to serialize and deserialize keys.

Would you accept a PR to add this feature?

[tarcieri]

Yes, that sounds fine