* Add unit test to confirm that supplementary items must be contained within actual sections
* [ASLayoutInspecting] Deprecate collectionView:numberOfSectionsForSupplementaryNodeOfKind:
* Add another test to dig deeper into UICollectionView
[ASRangeController] We're already on main thread, remove blocks
Make data source read-only, clarify what's asynchronous
[ASDataController] Clean up some interfaces
[ASDataController] A little more cleanup
[ASDataController] Cleanup
[ASDataController] Restore some changes, exit more often
[ASDataController] Use item counts that we already have rather than requerying them
[ASDataController] Revert weakifications
[ASDataController] Add a mechanism to measure how much work we avoided
