Adopt the new secret key interface

[burdges]

We should replace the loose public key interface we inherited from ed25519-dalek:

• https://docs.rs/schnorrkel/latest/schnorrkel/keys/struct.Keypair.html
• https://docs.rs/schnorrkel/latest/schnorrkel/keys/struct.SecretKey.html#method.sign

We'll basically make SecretKey be a keypair that contains the PublicKey. I'm already doing the ring VRF this way, so then we can backport stuff. It's kind an overhaul of the whole crate interface, but once someone asked me how to use schnorrkel with the randomness in merlin disabled, so likely worth doing.

[burdges]

We should decide if RistrettoBoth really makes sense. We could make it lazy but then we'd need interior mutability.