Prior to this commit, MerkleStore allowed the creation of Sparse Merkle
tree only with the maximum depth of 63. However, this doesn't fit the
Tiered Sparse Merkle tree requirements, as it will contain trees of
depth 16.

This commit adds the `depth` argument to the MerkleStore methods that
will create Sparse Merkle trees.

This commit introduces a wrapper structure to encapsulate the merkle
tree traversal.

related issue: #36

doc: benchmark hash functions

fix nits

fix: misc. nits

fix: nits and link

fix additional nits

fix: nits