Figure out how to create merkle proofs for CosmWasm `Map`s

Summary

CosmWasm stores each Map item under a separate (namespaced) key. This makes it complicated to prove the existence of multiple items in an 'all-encompassing' manner (i.e. covering all items).

We could store the list in a serialized Vec but that would lead to increased gas costs for checking duplicates, etc. and we would have to implement pagination ourselves. (This is the current impl in the https://github.com/informalsystems/bisenzone-cw-mvp)

A potential alternative is to use cw_storage_plus::Deque (although that also probably uses namespaced keys).

Acceptance Criteria

• Devise/Implement a better method to store and retrieve items on CosmWasm with proofs.

Start by reviewing the CosmWasm Map storage model and the cw_storage_plus::Deque alternative described in the issue, including the bisenzone-cw-mvp approach. Compare storage, pagination, duplicate-checking, gas, and proof requirements. Done means a documented or implemented method for storing and retrieving items with proofs that satisfies the acceptance criterion.