DKG Removals (#467)

* Update ValidatorSets with a remove_participant call

* Add DkgRemoval, a sign machine for producing the relevant MuSig signatures

* Don't use position-dependent u8s yet Public when removing validators from the DKG

* Add DkgRemovalPreprocess, DkgRemovalShares

Implementation is via a new publish_tributary_tx lambda.

This is code is a copy-pasted mess which will need to be cleaned up.

* Only allow non-removed validators to vote for removals

Otherwise, it's risked that the remaining validators fall below 67% of the
original set.

* Correct publish_serai_tx, which was prior publish_set_keys in practice

coordinator/src/tributary/db.rs

@@ -18,6 +18,7 @@ use crate::tributary::TributarySpec;
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Encode, Decode)]
 pub enum Topic {
   Dkg,
+  DkgRemoval([u8; 32]),
   SubstrateSign(SubstrateSignableId),
   Sign([u8; 32]),
 }
@@ -49,7 +50,10 @@ create_db!(
     DkgShare: (genesis: [u8; 32], from: u16, to: u16) -> Vec<u8>,
     PlanIds: (genesis: &[u8], block: u64) -> Vec<[u8; 32]>,
     ConfirmationNonces: (genesis: [u8; 32], attempt: u32) -> HashMap<Participant, Vec<u8>>,
+    RemovalNonces:
+      (genesis: [u8; 32], removing: [u8; 32], attempt: u32) -> HashMap<Participant, Vec<u8>>,
     CurrentlyCompletingKeyPair: (genesis: [u8; 32]) -> KeyPair,
+    DkgCompleted: (genesis: [u8; 32]) -> (),
     AttemptDb: (genesis: [u8; 32], topic: &Topic) -> u32,
     DataReceived: (genesis: [u8; 32], data_spec: &DataSpecification) -> u16,
     DataDb: (genesis: [u8; 32], data_spec: &DataSpecification, signer_bytes: &[u8; 32]) -> Vec<u8>,