Inline cloning the layers into a new Merkle.

This was a suggestion from Lee to keep from taking the lock twice.

arbitrator/prover/src/merkle.rs

@@ -199,11 +199,7 @@ fn new_layer(ty: MerkleType, layer: &[Bytes32], empty_hash: &'static Bytes32) ->
 
 impl Clone for Merkle {
     fn clone(&self) -> Self {
-        let leaves = if self.layers.lock().data.is_empty() {
-            vec![]
-        } else {
-            self.layers.lock().data[0].clone()
-        };
+        let leaves = self.layers.lock().data.get(0).cloned().unwrap_or_default();
         Merkle::new_advanced(self.ty, leaves, self.min_depth)
     }
 }