[contracts] The ability to verify a tally result on-chain

[weijiekoh]

Refer to: #91

When a 3rd-party contract wants to trustlessly prove that they know the final vote tally of vote option n from MACI contract m, they should be able to do the following:

1. Have access to the salt s and vote tally commitment c, which should be retrievable on-chain from m
2. Have access to the full vote tally r. Assume that the coordinator gives them this data off-chain.
3. Generate a Merkle proof: proof = genMerkleProof(n, r)
4. Show that hash(proof.root, s) == c
5. Show that genMerkleRoot(proof.elements, proof.indices) == proof.root