Clarify Registration and PartyId

[spaceships]

The UC security modelling in https://eprint.iacr.org/2021/916.pdf makes use of a PartyId.

Clarify how registration happens and the role of PartyId.

• KeyReg initialization should take a list of valid party IDs, or create them itself. PartyId is currently used as an index into the Merkle Tree as well as a global identifier for each party. The former could be chosen by KeyReg while the latter could be a hash of the public key.
• Address maliciously chosen PartyIds by ensuring the public keys are unique.

[markulf]

This seems to be an important passage in the paper regarding the key registration assumption:

The first ingredient is a key registration functionality that organizes the
participants’ stake; to minimize the assumptions placed on the setup of the
primitive we assume that key registration functionality is aware of the stake
of participants and invites them to register their cryptographic keys. Upon
termination of this phase the parties can retrieve those keys and organize them
in a Merkle tree (note that this Merkle tree organization can take place as part
of a trusted setup and hence need not encumber the parties computationally).

Ultimately this assumption will need to be satisfied by the protocol using Mithril, but we should clarify it as much as possible here and provide an API that makes it easy to satisfy it.

[abakst]

As #16 was merged, I believe we can consider this closed.