* Minor refactors in ASCollectionView and its private cell classes
- `_ASCollectionReusableView` and `_ASCollectionViewCell` no longer expose getters for their collection element and cell node properties. This is to make sure that clients can only obtain elements from the visible map which is always the source of truth.
- Since the map can return `nil` for an element request, it's much safer to check and avoid adding/removing a nil pointer to an `NSArray`.
- Since we use a special way to check whether an object of kind of `_ASCollectionViewCell` or `_ASCollectionReusableView`, based on the assumption that these classes are subclass restricted, I added cast-or-return macros in the header files, closer to where the restrictions are declared.
* Add ASDynamicCastStrict
* Add element and node properties back to _ASCollectionReusableView and _ASCollectionViewCell
* Assert unexpected nil elements
* Always mark an element visible even if it is backed by an UIKit / non-_ASCollection* view
* Fix typo
* Remove unnecessary changes
* Dump mistakes
* Update CHANGELOG
* Can't track visibility of elements backed by non-_AS views
* no message
* Go further
* Make the symbols public so that apps actually build
* Move ASAvailability into the umbrella header
* Remove duplicate define
* Put the file back where it was in the list
* Revert "Put the file back where it was in the list"
This reverts commit 6a80c15b5b5efe5ff39812a018114e8bdc1dc0cf.
