Complete overhaul of ASFlowLayoutController.

Introduced ASIndexPath for efficient handling of index paths in C++ vectors,
while maintaining the readability of ".section" and ".row" instead of
".first" and ".second" inside of complicated business logic.

Confirmed that the working range calls are firing appropriately during
ASTableViewStressTest, including the deallocation of the rich text placeholders
provided by ASTextNode.

AsyncDisplayKit/Details/ASCollectionViewLayoutController.h

@@ -14,6 +14,5 @@
 @interface ASCollectionViewLayoutController : ASAbstractLayoutController
 
 - (instancetype)initWithCollectionView:(ASCollectionView *)collectionView;
-- (instancetype)initWithScrollView:(UIScrollView *)scrollView collectionViewLayout:(UICollectionViewLayout *)layout;
 
 @end