feature: store intermediary reduce values

[mikeal]

There’s two primary use cases I’m thinking of.

1. We can migrate the SpareArray to storing integer deltas for keys and we can put the sum in each branch key. This way, we can take SQL proofs for append-only INSERT statements and commute them together. We just take the number of appended entries in each tree and we can glue them all together with minimal computation and churn.
2. We can make a DBNumberIndex that stores the sum and count reduce values for each part of the tree. Then we can drastically speed up SQL statements for many math operations.