Subregistry commitments

Based off of discussion with @ericxtang and @j0sh

Suppose Os can be grouped into bundles based on a category such as geographic location. From a global registry of Os we can derive a number of subregistries for each of these categories. Each of these subregistries would have a fixed number of slots for Os. If we define a global `orchestratorMaxFloat` as mentioned in #18 then we can derive the max float for the entire subregistry as `subregistryMaxFloat = max subregistry size * orchestratorMaxFloat`.

A B can commit to subregistries (the word "subscribe" has also been used, but I'll stick with the terminology from the recipient set commitment construction for now) in order to work with Os in the subregistries. B does not have to worry about the actual membership changes in the subregistry as long as the subregistry has a fixed number of slots - if this is the case then `subregistryMaxFloat` will always remain the same given a constant global `orchestratorMaxFloat`.

B's reserve is sliced into allocations for each committed subregistry. Then each committed subregistry allocation is further sliced into allocations for each O in the subregistry.

The issue describes a research proposal for subregistry commitments but names no files, tests, or implementation entry points. First clarify the intended design and locate the existing orchestratorMaxFloat and recipient set commitment work, then define the expected behavior for subregistry membership, fixed slots, and reserve allocations.

Written by the indexing model from the issue text.