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/error.rs

@@ -1,6 +1,6 @@
 use std::{backtrace::Backtrace, fmt::Debug};
 
-use crate::middleware::{DynError, Statement, StatementTmpl, Value};
+use crate::middleware::{BackendError, Statement, StatementTmpl, Value};
 
 pub type Result<T, E = Error> = core::result::Result<T, E>;
 
@@ -41,7 +41,7 @@ pub enum Error {
     #[error(transparent)]
     Infallible(#[from] std::convert::Infallible),
     #[error(transparent)]
-    Backend(#[from] Box<DynError>),
+    Backend(#[from] BackendError),
     #[error(transparent)]
     Middleware(#[from] crate::middleware::Error),
 }