Add self support to Reducer

[ecton]

After optimizing count() in Bonsai after adding reduce() here, I looked at storing the view reductions in Nebari's indexes ( khonsulabs/bonsaidb#76). I realized that for BonsaiDb to be able to use a View's reduce function, the Reducer needs a self that can store the view.

The problem is, where does the reducer get instantiated and stored? Or is it better to refactor the reducer to be the Root itself?