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

related issue: #36

A Merkle path is a vector of nodes, regardless of the Merkle tree
implementation.

This commit introduces an encapsulation for such vector, also to provide
functionality that is common between different algorithms such as
opening verification.

related issue: #36

This commit moves the previous implementation of `SparseMerkleTree` from
miden-core to this crate.

It also include a couple of new tests, a bench suite, and a couple of
minor fixes. The original API was preserved to maintain compatibility
with `AdviceTape`.

closes #21