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/basetypes.rs

@@ -35,6 +35,6 @@
 /// then the Value, Hash and F types would come from the plonky3 backend.
 #[cfg(feature = "backend_plonky2")]
 pub use crate::backends::plonky2::basetypes::{
-    hash_fields, hash_str, hash_value, Hash, Value, EMPTY, F, HASH_SIZE, NULL, SELF_ID_HASH,
-    VALUE_SIZE,
+    hash_fields, hash_str, hash_value, Hash, Value, EMPTY_HASH, EMPTY_VALUE, F, HASH_SIZE,
+    SELF_ID_HASH, VALUE_SIZE,
 };