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/middleware/mod.rs

@@ -23,7 +23,7 @@ impl fmt::Display for PodId {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if *self == SELF {
             write!(f, "self")
-        } else if self.0 == NULL {
+        } else if self.0 == EMPTY_HASH {
             write!(f, "null")
         } else {
             write!(f, "{}", self.0)
@@ -185,7 +185,7 @@ impl Pod for NonePod {
         true
     }
     fn id(&self) -> PodId {
-        PodId(NULL)
+        PodId(EMPTY_HASH)
     }
     fn pub_statements(&self) -> Vec<Statement> {
         Vec::new()