Slash reports (#523)

* report_slashes plumbing in Substrate

Notably delays the SetRetired event until it provides a slash report or the set
after it becomes the set to report its slashes.

* Add dedicated AcceptedHandover event

* Add SlashReport TX to Tributary

* Create SlashReport TXs

* Handle SlashReport TXs

* Add logic to generate a SlashReport to the coordinator

* Route SlashReportSigner into the processor

* Finish routing the SlashReport signing/TX publication

* Add serai feature to processor's serai-client

processor/src/batch_signer.rs

@@ -213,6 +213,10 @@ impl<D: Db> BatchSigner<D> {
         panic!("BatchSigner passed CosignSubstrateBlock")
       }
 
+      CoordinatorMessage::SignSlashReport { .. } => {
+        panic!("Cosigner passed SignSlashReport")
+      }
+
       CoordinatorMessage::SubstratePreprocesses { id, preprocesses } => {
         let (session, id, attempt) = self.verify_id(&id).ok()?;