Use proper messages for ValidatorSets/InInstructions pallet

Provides a DST, and associated metadata as beneficial.

Also utilizes MuSig's context to session-bind. Since set_keys_messages also
binds to set, this is semi-redundant, yet that's appreciated.

substrate/in-instructions/primitives/src/lib.rs

@@ -79,3 +79,8 @@ impl Zeroize for SignedBatch {
     self.signature.as_mut().zeroize();
   }
 }
+
+/// The message for the batch signature.
+pub fn batch_message(batch: &Batch) -> Vec<u8> {
+  [b"InInstructions-batch".as_ref(), &batch.encode()].concat()
+}