Prior to this commit, there was an internal procedure with the merkle
trees to compute empty sub-tree for arbitrary depths.
However, this isn't ideal as this code can be reused in any merkle
implementation that uses RPO as backend.
This commit introduces a structure that will generate these empty
subtrees values.
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
