* Smt::inner_nodes

* Add conversion Smt -> MerkleStore

* add docstring to `Smt`

* add to docstring

* fmt

* add `leaves()` method to `Smt`

* add `kv_pairs` functions

* rewrite `into_elements()` in terms of `into_kv_pairs()`

* change docstring

* tests: fix typos
* full.rs: fix typo
* CONTRIBUTING: fix typo

feat: implement additional functionality for the PartialMerkleTree

Consider the case of a MMR with one entry, and a new entry is being
added. Both of these values are quite unique, they are at the same time
the root and only leaf of their corresponding tree. Currently this
representation is not supported by the [MerkleStore], so the leaves are
not in it. Once the two values are merged, they both become leaves of a
new tree under the new parent, and the existing validation didn't permit
that promotion from happening.

This lifts the validation, and changes the method to clarify that not
only `root` are being merged, by arbitrary nodes of a tree (leafs,
internal, or roots), with arbitrary mixing of each.

This commit introduces `get_leaf_depth`, a tiered SMT helpers that will
retrieve the depth of a leaf for a given root, capped by `64`.

closes #119

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.

The path returned by `EmptySubtreeRoots` starts at the root, and goes to
the leaf. The MerkleStore constructor assumed the other direction, so
the parent/child hashes were reversed.

This fixes the bug and adds a test.