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/tests/tributary/mod.rs

@@ -202,6 +202,17 @@ fn serialize_transaction() {
     random_signed_with_nonce(&mut OsRng, 2),
   ));
 
+  {
+    let mut key = [0; 32];
+    OsRng.fill_bytes(&mut key);
+    test_read_write(Transaction::DkgRemovalPreprocess(random_sign_data(&mut OsRng, key, 0)));
+  }
+  {
+    let mut key = [0; 32];
+    OsRng.fill_bytes(&mut key);
+    test_read_write(Transaction::DkgRemovalShare(random_sign_data(&mut OsRng, key, 1)));
+  }
+
   {
     let mut block = [0; 32];
     OsRng.fill_bytes(&mut block);