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/frontend/mod.rs

@@ -564,7 +564,7 @@ impl MainPodBuilder {
             statements: &statements,
             operations: &operations,
             public_statements: &public_statements,
-            vds_set: self.vd_set.clone(),
+            vd_set: self.vd_set.clone(),
         };
         let pod = prover.prove(&self.params, &self.vd_set, inputs)?;