Further reorganization

[frankmcsherry]

Ongoing reorganization of traits and types.

1. The Batcher trait is moved to a top level batcher module, elevating it as a core concept (the transition from stream to ledger).
2. The Cursor trait and its cohort move to operators/cursor/ where they live as a coherent opinion about how to implement operators, but not much more (ideally).
3. The "reference" reduce implementation is removed, as it turns out to be only an alternate implementation that isn't any more adherent to the formal/ description, just "coarser grained".
4. The cursor use from the history replaying mechanisms is extracted to cursor/history.rs.