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.

Prior to this commit, we limited the constants count to 64 for the empty
subtrees depth computation. This is a hard-assumption that every tree of
Miden will have a depth up to 64 - and will cause undefined behavior if
it doesn't.

With the introduction of `MerkleStore::merge_roots` and the deprecation
of `mtree_cwm` instruction from the VM, this assumption is broken and
the user might end with trees with depth greater than 64. This broken
assumption could lead to attack vectors.

We can easily fix that by extending the pre-computed hashes list to the
maximum of `u8` (i.e. 255). This will have zero impact on functionality,
and will be completely safe to use without hard assumptions.

Example formatted MerkleTree:

```
880abe452320966617646e7740b014954300f19a28780a0889d62ff33f4b0534
  1ade1369091efa31201e9b60c9c28874d0ddce5362b335135a6bb4c917285983
  3e60a9c843b4bb19f7a0572102e6507195f5240767a396335fd21981b048b807
    0100000000000000000000000000000000000000000000000000000000000000
    0200000000000000000000000000000000000000000000000000000000000000
    0300000000000000000000000000000000000000000000000000000000000000
    0400000000000000000000000000000000000000000000000000000000000000
```

Example formatted MerklePath:

```
[0400000000000000000000000000000000000000000000000000000000000000, 1ade1369091efa31201e9b60c9c28874d0ddce5362b335135a6bb4c917285983]
```

And unit tests for each constructor type.

Returning tuples is often confusing as they don't convey meaning and it
should be used only when there is no possible ambiguity.

For `MerkleStore`, we had a couple of tuples being returned, and reading
the implementation was required in order to distinguish if they were
leaf values or computed roots.

This commit introduces two containers that will self-document these
returns: `RootPath` and `ValuePath`. It will also update `set_node` to
return both the new root & the new path, so we can prevent duplicated
traversals downstream when updating a node (one to update, the second to
fetch the new path/root).

The store builds the path from root to leaf, this updates the code to
return a path from leaf to root, as it is done by the other structures.

This also added custom error for missing root.

Prior to this commit, the MerkleStore panicked under certain bounds. It
will prevent such panics by using checked operations.

ilog2, for instance, will panic when the operand is zero. However, there
is a documentation rule enforcing the merkle tree to be size at least 2.
If this rule is checked, then the panic is impossible.

This commit introduce common derive proc macros to MerkleStore. These
are required downstream as the in-memory storage can be cloned.

It also introduces constructors common to the other types of the crate
that will help to build a merkle store, using a build pattern.