Update Update and substrate_signers to [u8; 32] from Vec<u8>

A commit made while testing moved them from network-key-indexed to
Substrate-key-indexed. Since Substrate keys have a fixed-length, fitting within
the Copy boundary, there's no reason for it to not use an array.

processor/messages/src/lib.rs

@@ -176,7 +176,7 @@ pub mod substrate {
 
   #[derive(Clone, PartialEq, Eq, Debug, Zeroize, Serialize, Deserialize)]
   pub enum ProcessorMessage {
-    Update { key: Vec<u8>, batch: SignedBatch },
+    Update { key: [u8; 32], batch: SignedBatch },
   }
 }