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.

Implement addBatchInDisk for big trees, which does not puts the tree in
memory, and works directly over the db data, parallelizing for each CPU.

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).

Add circom test with circuit for CircomVerifierProofs, which allows to
automatically check that the data generated from arbo matches the circom
circuit of a SMTVerifierProof.
Added also GHA workflow to test the circuits with the output of arbo
code.