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