Enhancement to support configurable hashing algorithms

[jimthematrix]

This is useful in situations when the SMT construct is used without needing ZKPs, for instance when used by the notary based privacy token implementation in LFDT's Paladin project.

The goal is first of all making the current Poseidon hash the default, such that existing clients using the npm module does not need to change the source code at all after the dependency upgrade.

To use an alternative hashing algo, pass in the optional parameter when constructing the merkle tree storage object:

new LocalStorageDB(str2Bytes(prefix), HashAlgorithm.Keccak256);

or when verifying the proof:

await verifyProof(await mt.root(), proof, BigInt('33'), BigInt('44'), HashAlgorithm.Keccak256))

As of this PR, only keccak256 is added as the new hashing algorithm.

The unit tests are cross-checked with unit tests in this PR: iden3/contracts#402