Serialization for Plonky2 Signed and Main PODs (#234)

* WIP

* WIP

* Working serialization for both Mock and Plonky2 versions of Signed and Main Pods

* Restore useful comment about serialized_proof()

* Use plonky2 serialization for signatures and proofs

* Add schema renames for Serialized SignedPod/MainPod types

* Break out utility function for generating common circuit data

* Review feedback fixes

src/backends/plonky2/mock/signedpod.rs

@@ -59,6 +59,10 @@ impl MockSignedPod {
     pub(crate) fn new(id: PodId, signature: String, kvs: HashMap<Key, Value>) -> Self {
         Self { id, signature, kvs }
     }
+
+    pub fn signature(&self) -> String {
+        self.signature.clone()
+    }
 }
 
 impl MockSignedPod {
@@ -131,7 +135,7 @@ impl Pod for MockSignedPod {
     }
 
     fn serialized_proof(&self) -> String {
-        self.signature.to_string()
+        serde_json::to_string(&self.signature).unwrap()
     }
 }