ThresholdNLeafs defines the threshold number of leafs in the tree that
determines if AddBatch will work in memory or in disk. It is defined
when calling NewTree, and if set to 0 it will work always in disk.
Add checks that the key is not bigger than maximum key length for the tree
maxLevels size, where maximum key len = ceil(maxLevels/8).
This is because if the key bits length is bigger than the maxLevels of the
tree, two different keys that their difference is at the end, will collision in
the same leaf of the tree (at the max depth).
Circom Verifier Proofs allow to verify through a zkSNARK proof the
inclusion/exclusion of a leaf in a tree. This commit adds the needed
code in go to generate the circuit inputs for a CircomVerifierProof.
