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.
Qyriad
ca3c79e818