put 'prospective' first in tests for prospective commit

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, making the prospective hash logic a
combination of the normal insertion logic and the normal hash logic. But
because we're only interested in the hash and not the intermediate
value, we can skip allocations and sorts for the payload itself.

series

@@ -1 +1 @@
-Subproject commit 1662a0a8daceb5269329348d8c0b5431a1ce3198
+Subproject commit 4d1f33ab6775297ae617e353acbe8388e1c36fc8