[crypto] next steps

[rt2zz]

1. check if cryptoVerify will work with strings (or if it requires a buffer)
2. look into adding explicit serialize / rehydrate to cryptoCreateKeypair and cryptoSign
3. write tests to cover all of this

[rt2zz]

one more reason not to go back to msgpack / buffers is that this will likely make compatibility more difficult with potential non-nodejs clients.