Add SignedPod verification circuit (SignedPodVerifyGadget) (#170)

* add boolean selector to the MerkleProofGadget, to allow skipping proof verifications when all the slots are not used (eg. in the SignedPod circuit)

* move existing signedpod's circuits draft to its own file

* implement SignedPodVerify circuit

src/backends/plonky2/mock/signedpod.rs

@@ -56,17 +56,7 @@ impl MockSignedPod {
 
 impl Pod for MockSignedPod {
     fn verify(&self) -> Result<()> {
-        // 1. Verify type
-        let value_at_type = self.dict.get(&hash_str(KEY_TYPE).into())?;
-        if Value::from(PodType::MockSigned) != value_at_type {
-            return Err(anyhow!(
-                "type does not match, expected MockSigned ({}), found {}",
-                PodType::MockSigned,
-                value_at_type
-            ));
-        }
-
-        // 2. Verify id
+        // 1. Verify id
         let mt = MerkleTree::new(
             MAX_DEPTH,
             &self
@@ -84,6 +74,16 @@ impl Pod for MockSignedPod {
             ));
         }
 
+        // 2. Verify type
+        let value_at_type = self.dict.get(&hash_str(KEY_TYPE).into())?;
+        if Value::from(PodType::MockSigned) != value_at_type {
+            return Err(anyhow!(
+                "type does not match, expected MockSigned ({}), found {}",
+                PodType::MockSigned,
+                value_at_type
+            ));
+        }
+
         // 3. Verify signature
         let pk_hash = self.dict.get(&hash_str(KEY_SIGNER).into())?;
         let signature = format!("{}_signed_by_{}", id, pk_hash);