Add verifier-datas tree (set) & in-circuit verification (#274)

* containers: add method to create new {Dict,Set,Array} with custom max_depth

* add vds_tree computation, update tree circuit interface

* add VDTree struct, add DEFAULT_VD_TREE, integrate it with MainPod,EmptyPod,frontend,etc.

* adapt frontend/serialization tests to new containers field (max_depth)

* adapt interfaces to allow using custom vd_tree in frontend & backend constructors

* rename VDTree to VDSet (and derivate namings too)

* containers 'new' always with param 'max_depth', use params.max_depth_mt_containers instead of the global constant MAX_DEPTH

* adapt after rebasing the branch to main latest changes

* apply review suggestions from @ed255

* use emptypod vd_mt_proofs (using vd_set as circuit input), merge the two existing set_targets methods of MainPodVerifyTarget

* document VDSet & vds_root

src/backends/plonky2/circuits/signedpod.rs

@@ -41,7 +41,7 @@ impl SignedPodVerifyGadget {
         let mut mt_proofs = Vec::new();
         for _ in 0..self.params.max_signed_pod_values {
             let mt_proof = MerkleProofExistenceGadget {
-                max_depth: self.params.max_depth_mt_gadget,
+                max_depth: self.params.max_depth_mt_containers,
             }
             .eval(builder)?;
             builder.connect_hashes(id, mt_proof.root);