Merkleproof verify circuit (#143)

* merkletree: add keypath circuit

* merkletree-circuit: implement proof of existence verification in-circuit

* parametrize max_depth at the tree circuit

* Constrain selectors in-circuit

* implement merketree nonexistence proof circuit, and add edgecase tests

* add non-existence proofs documentation in the mdbook, mv EMPTY->EMPTY_VALUE & NULL->EMPTY_HASH, dependency clean and public exposure methods

* review comments, some extra polishing and add a test that expects wrong proofs to fail

* Add circuit to check only merkleproofs-of-existence

With this, the merkletree_circuit module offers two different circuits:
- `MerkleProofCircuit`: allows to verify both proofs of existence and proofs
non-existence with the same circuit.
- `MerkleProofExistenceCircuit`: allows to verify proofs of existence only.

In this way, if only proofs of existence are needed,
`MerkleProofExistenceCircuit` should be used, which requires less amount
of constraints than `MerkleProofCircuit`.

* Code review

---------

Co-authored-by: Ahmad <root@ahmadafuni.com>

src/backends/plonky2/mock_signed.rs

@@ -110,7 +110,7 @@ pub mod tests {
     use super::*;
     use crate::constants::MAX_DEPTH;
     use crate::frontend;
-    use crate::middleware::{self, F, NULL};
+    use crate::middleware::{self, EMPTY_HASH, F};
 
     #[test]
     fn test_mock_signed_0() -> Result<()> {
@@ -137,7 +137,7 @@ pub mod tests {
         assert!(!bad_pod.verify());
 
         let mut bad_pod = pod.clone();
-        let bad_kv = (hash_str(KEY_SIGNER).into(), Value(PodId(NULL).0 .0));
+        let bad_kv = (hash_str(KEY_SIGNER).into(), Value(PodId(EMPTY_HASH).0 .0));
         let bad_kvs_mt = &bad_pod
             .kvs()
             .into_iter()