Introduce ASCellNodeDelegate

- Cell node automatically notifies the delegate after a relayout (via -setNeedsLayout) that results in a new size. Confirming to ASCellNodeDelegate; ASTableView and ASCollectionView reload the calling cell upon notifications. These views automatically set themselves as delegate of every node.
- The result is that ASCellNode subclasses don't need to manually notify the containing view. Thus, `-relayoutItemAtIndexPath` and `-relayoutRowAtIndexPath` are removed.
- Kittens example is updated to reflect the change.

AsyncDisplayKit/ASCollectionView.mm

@@ -130,7 +130,7 @@ static BOOL _isInterceptedSelector(SEL sel)
 #pragma mark -
 #pragma mark ASCollectionView.
 
-@interface ASCollectionView () <ASRangeControllerDelegate, ASDataControllerSource> {
+@interface ASCollectionView () <ASRangeControllerDelegate, ASDataControllerSource, ASCellNodeDelegate> {
   _ASCollectionViewProxy *_proxyDataSource;
   _ASCollectionViewProxy *_proxyDelegate;
 
@@ -457,14 +457,6 @@ static BOOL _isInterceptedSelector(SEL sel)
   [_dataController reloadRowsAtIndexPaths:indexPaths withAnimationOptions:kASCollectionViewAnimationNone];
 }
 
-- (void)relayoutItemAtIndexPath:(NSIndexPath *)indexPath
-{
-  ASDisplayNodeAssertMainThread();
-  ASCellNode *node = [self nodeForItemAtIndexPath:indexPath];
-  [node setNeedsLayout];
-  [super reloadItemsAtIndexPaths:@[indexPath]];
-}
-
 - (void)moveItemAtIndexPath:(NSIndexPath *)indexPath toIndexPath:(NSIndexPath *)newIndexPath
 {
   ASDisplayNodeAssertMainThread();
@@ -663,6 +655,7 @@ static BOOL _isInterceptedSelector(SEL sel)
 {
   ASCellNode *node = [_asyncDataSource collectionView:self nodeForItemAtIndexPath:indexPath];
   ASDisplayNodeAssert([node isKindOfClass:ASCellNode.class], @"invalid node class, expected ASCellNode");
+  node.delegate = self;
   return node;
 }
 
@@ -906,4 +899,15 @@ static BOOL _isInterceptedSelector(SEL sel)
   }
 }
 
+#pragma mark - ASCellNodeDelegate
+
+- (void)node:(ASCellNode *)node didRelayoutToNewSize:(CGSize)newSize suggestedAnimation:(ASCellNodeAnimation)animation
+{
+  ASDisplayNodeAssertMainThread();
+  NSIndexPath *indexPath = [self indexPathForNode:node];
+  if (indexPath != nil) {
+    [super reloadItemsAtIndexPaths:@[indexPath]];
+  }
+}
+
 @end