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

doc: benchmark hash functions

fix nits

fix: misc. nits

fix: nits and link

fix additional nits

fix: nits