Complete the verification in MainMockPod (#302)

- Update the `RecursivePod` trait to return `vd_set` instead of `vds_root`
  - A native verifier requires the entire set to reason about the circuits that have been used in the recursive tree
- Implement serialization/deserialization for `VDSet`
- Remove `DynError` and use `BackendError` instead for middleware functions that wrap or define trait functions implemented in the backend.  This is based on the fact that we will only have a single backend enabled at a time, so there's no need for a `dyn Error`
  - Move the implementations of `_verify` functions to `verify` and similarly for `_prove`
- Complete the verification of a MockMainPod: the verification of input pods was missing.  The inclusion of these input pods in the serialization was also missing.  With this change a `MockMainPod` will grow after each recursion.  This was expected from the design but was not the case because of the missing recursive native verification implementation.

* apply feedback from @arnaucube

src/backends/plonky2/circuits/mainpod.rs

@@ -1528,7 +1528,7 @@ impl InnerCircuit for MainPodVerifyTarget {
         }
         // Padding
         if input.recursive_pods_pub_self_statements.len() != self.params.max_input_recursive_pods {
-            let empty_pod = EmptyPod::new_boxed(&self.params, input.vds_set.root());
+            let empty_pod = EmptyPod::new_boxed(&self.params, input.vds_set.clone());
             let empty_pod_statements = empty_pod.pub_statements();
             for i in
                 input.recursive_pods_pub_self_statements.len()..self.params.max_input_recursive_pods