[ASCollectionView] Finish support for interoperability with base-class UICollectionViewCells.

This also supports supplementary nodes. It builds off of Adlai's .interop flag but makes necessary
improvements for all of the delegate methods to work in practice with heterogenous cell types.

examples/CustomCollectionView/Sample/MosaicCollectionViewLayout.h

@@ -28,6 +28,9 @@
 @property (assign, nonatomic) UIEdgeInsets interItemSpacing;
 @property (assign, nonatomic) CGFloat headerHeight;
 
+- (CGSize)itemSizeAtIndexPath:(NSIndexPath *)indexPath;
+- (CGSize)headerSizeForSection:(NSInteger)section;
+
 @end
 
 @protocol MosaicCollectionViewLayoutDelegate <ASCollectionDelegate>
@@ -35,7 +38,3 @@
 - (CGSize)collectionView:(UICollectionView *)collectionView layout:(MosaicCollectionViewLayout *)layout originalItemSizeAtIndexPath:(NSIndexPath *)indexPath;
 
 @end
-
-@interface MosaicCollectionViewLayoutInspector : NSObject <ASCollectionViewLayoutInspecting>
-
-@end