Progress on the MainPod circuit (#159)

* feat: add SignedPodVerify test

* unify circuits style

* more clear sizes

* get operation_verify test working

* be consistent with names

src/backends/plonky2/mock_main/mod.rs

@@ -260,7 +260,7 @@ impl MockMainPod {
                 .map(|mid_arg| Self::find_op_arg(statements, mid_arg))
                 .collect::<Result<Vec<_>>>()?;
             Self::pad_operation_args(params, &mut args);
-            operations.push(Operation(op.predicate(), args));
+            operations.push(Operation(op.op_type(), args));
         }
         Ok(operations)
     }