This commit implements SparseMerkleTree::insert_ensure_root(), a version
of SparseMerkleTree::insert() that prevents modification of the tree if
the insert turns out to not result in the correct tree root (i.e., the
transaction is not valid).
This is an initial step towards issue 222.
As a logical next step, generalizing this to validation of inserting
multiple values at once will directly enable removing Merkle tree clones
in miden-node InnerState::apply_block().
For further work, as we generalize pre-validation for mutations we will
want a separate type to represent an arbitrary set of prospective
mutations on the Merkle tree and their validity, like the `ChangeSet`
type suggested in the issue.
This commit implements 'prospective leaf hashing' -- computing what the
hash of a sparse Merkle tree leaf *would* be for a key-value insertion
without actually performing that insertion.
For SimpleSmt, this is trivial, since the leaf hash and its payload are
the same.
For the full Smt, the new leaf payload (and thus, its hash) depend on
the existing payload in that leaf. Since almost all leaves are very
small, we can just clone the leaf and modify a copy.
Future work could optimize the uncommon SmtLeaf::Multiple case and
avoid a heap clone by combining the normal insertion logic and the
normal hash logic without an intermediate leaf value.
