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/main.rs

@@ -199,7 +199,6 @@ async fn handle_processor_message<D: Db, P: P2p>(
       key_gen::ProcessorMessage::GeneratedKeyPair { id, .. } => Some(id.session),
       key_gen::ProcessorMessage::Blame { id, .. } => Some(id.session),
     },
-    // TODO: Review replacing key with Session in messages?
     ProcessorMessage::Sign(inner_msg) => match inner_msg {
       // We'll only receive InvalidParticipant/Preprocess/Share if we're actively signing
       sign::ProcessorMessage::InvalidParticipant { id, .. } => Some(id.session),