Improve Our Handling of Flow Layout Headers/Footers (#2939)

* Rejigger our flow layout supplementary support to make more sense.

* Support old protocol

* Update

* Update deprecation message

* Undeprecate insetForSection method, because it actually _does_ still work

* Update the tests

* Remove irrelevant junk

* Remove cast, add pragma

examples/CatDealsCollectionView/Sample/LoadingNode.m

@@ -39,10 +39,6 @@ static CGFloat kFixedHeight = 200.0f;
 #pragma mark -
 #pragma mark ASCellNode.
 
-+ (CGFloat)desiredHeightForWidth:(CGFloat)width {
-  return kFixedHeight;
-}
-
 - (instancetype)init
 {
   if (!(self = [super init]))
@@ -61,10 +57,6 @@ static CGFloat kFixedHeight = 200.0f;
   return self;
 }
 
-- (void)layout {
-  [super layout];
-}
-
 - (ASLayoutSpec *)layoutSpecThatFits:(ASSizeRange)constrainedSize
 {
   ASCenterLayoutSpec *centerSpec = [[ASCenterLayoutSpec alloc] init];