* feat(smt): impl constructing leaves that don't yet exist

This commit implements 'prospective leaf construction' -- computing
sparse Merkle tree leaves for a key-value insertion without actually
performing that insertion.

For SimpleSmt, this is trivial, since the leaf type is simply the value
being inserted.

For the full Smt, the new leaf payload depends on the existing payload
in that leaf. Since almost all leaves are very small, we can just clone
the leaf and modify a copy.

This will allow us to perform more general prospective changes on Merkle
trees.

* feat(smt): export get_value() in the trait

* feat(smt): implement generic prospective insertions

This commit adds two methods to SparseMerkleTree: compute_mutations()
and apply_mutations(), which respectively create and consume a new
MutationSet type. This type represents as set of changes to a
SparseMerkleTree that haven't happened yet, and can be queried on to
ensure a set of insertions result in the correct tree root before
finalizing and committing the mutation.

This is a direct step towards issue 222, and will directly enable
removing Merkle tree clones in miden-node InnerState::apply_block().

As part of this change, SparseMerkleTree now requires its Key to be Ord
and its Leaf to be Clone (both bounds which were already met by existing
implementations). The Ord bound could instead be changed to Eq + Hash,
if MutationSet were changed to use a HashMap instead of a BTreeMap.

* chore(smt): refactor empty node construction to helper function

* add accessors

* fmt

* comments

* add conversion for `SmtLeaf`

* introduce `SmtProof` scaffolding

* implement `verify_membership()`

* SmtLeaf: knows its index

* `SmtLeaf::index`

* `SmtLeaf::get_value()` returns an Option

* fix `verify_membership()`

* impl `SmtProof::get`

* impl `into_parts()`

* `SmtProof::compute_root`

* use `SmtProof` in `Smt::open`

* `SmtLeaf` constructors

* Vec

* impl `Error` for `SmtLeafError`

* fix std Error

* move Word/Digest conversions to LeafIndex

* `SmtProof::new()` returns an error

* `SparseMerkleTree::path_and_leaf_to_opening`

* `SmtLeaf`: serializable/deserializable

* `SmtProof`: serializable/deserializable

* add tests for SmtLeaf serialization

* move `SmtLeaf` to submodule

* use constructors internally

* fix docs

* Add `Vec`

* add `Vec` to tests

* no_std use statements

* fmt

* `Errors`: make heading

* use `SMT_DEPTH`

* SmtLeaf single case: check leaf index

* Multiple case: check consistency with leaf index

* use `pub(super)` instead of `pub(crate)`

* use `pub(super)`

* `SmtLeaf`: add `num_entries()` accessor

* Fix `SmtLeaf` serialization

* improve leaf serialization tests

* 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

fixes: #258

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

* recompute_nodes_from_indeX_to_root

* MerkleError variant

* set_subtree

* test_simplesmt_set_subtree

* test_simplesmt_set_subtree_entire_tree

* test

* set_subtree: return root

When a prefix is pushed to the depth 64, the entry list includes only
the values different than ZERO. This is required, since each block
represents a 2^192 values.

The bug was in the proof membership code, that failed to handle the case
of a key that was not in the list, because the depth is 64 and the value
was not set.