This commit adds a type, MutationSet, which represents a 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.
Additionally, as MutationSet is a generic type
which works on any type that implements SparseMerkleTree, but is
intended for public API use, the SparseMerkleTree trait and InnerNode
type have been made public so MutationSet can be used outside of this
crate.
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.
