Delegated signatures

The multiple-signers facility of Fae allows various parties to protect their interests by restricting contract calls so that they are required to examine the transaction before allowing execution. This does require that the specific public key that is given this authority is known at the time the contract is created, and therefore disallows the parties from assigning separate agents who are authorized to sign transactions in their capacities.

There could be a new primitive, `delegate :: (MonadFae m) => PublicKey -> String -> m ()`, that fetches a signature by name and checks that it has delegate authority from the given public key. This authority would be signified by augmenting the delegates' public keys with signatures (of that key) by the delegating parties; if the signature checks out against the provided public key, then the delegate's public key has authority.

This suggests implementing a certificate-chain system, but that seems excessive right now.