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

substrate/validator-sets/primitives/src/lib.rs

@@ -98,9 +98,14 @@ pub fn musig_key(set: ValidatorSet, set_keys: &[Public]) -> Public {
   Public(dkg::musig::musig_key::<Ristretto>(&musig_context(set), &keys).unwrap().to_bytes())
 }
 
+/// The message for the remove_participant signature.
+pub fn remove_participant_message(set: &ValidatorSet, removed: Public) -> Vec<u8> {
+  (b"ValidatorSets-remove_participant", set, removed).encode()
+}
+
 /// The message for the set_keys signature.
 pub fn set_keys_message(set: &ValidatorSet, key_pair: &KeyPair) -> Vec<u8> {
-  [b"ValidatorSets-key_pair".as_ref(), &(set, key_pair).encode()].concat()
+  (b"ValidatorSets-set_keys", set, key_pair).encode()
 }
 
 /// For a set of validators whose key shares may exceed the maximum, reduce until they equal the