Cache `all_local_results` binding to avoid event explosion

* Previously, every render of a `with_direct_mappings_as` mustache
  helper would create a new multi-loader binding, which means more
  things listening to events, and propagating changes through unused
  lists.  Cache this binding and use one `all_local_results` binding per
  instance.
Merge pull request #1216 from reciprocity/feature/cache-all-local-results-binding

Cache `all_local_results` binding to avoid event explosion