musig: integration with Predicate Tree

[oleganza]

Currently Predicate::Key wraps a VerificationKey, so all information about the signing parties is erased.

Let's replace it with Multikey, so that we can preserve information for signing.

• [ ] Use Multikey instead of VerificationKey in the Predicate::Key variant.
• [ ] Make VerificationKey convertible Into<Multikey>
• [x] Make sure Multikey over a single key uses 1 as a delinearization factor (a_0), not a random one. This is needed for compatibility with raw VerificationKey.