Add bitfield signed messages?

[burdges]

We could add some analog of the bitfield signed message stuff from the BLS crate: https://github.com/w3f/bls/blob/master/src/bit.rs

Right now, you must implement it yourself using AggregatePublicKey::public_key, but you also cannot really screw it up since you cannot merge signatures like in BLS, so I'm not sure it's really critical here.