Integrate new layout nodes to the framework.

- Introduce ASLayoutNode and its subclasses.
- ASDisplayNode measures its ASLayoutNode and cache the result (ASLayout). Calculated size and position of each subnode can be retrieved from the calculated layout.
- Custom nodes need to override -layoutNodeThatFits:(CGSize) instead of -calculateSizeThatFits:(CGSize).
- Custom nodes do not need to layout its subnodes (in -layout:) anymore. ASDisplayNode can handle the job most of the time, by walking through its layout tree.
- ASCompositeNode is used to embed (display) subnodes to a node's layout. That way, each subnode will also be measured while the parent node is measuring. And the parent node knows where its subnodes are within its layout.

AsyncDisplayKit/Private/ASDisplayNodeInternal.h

@@ -17,9 +17,8 @@
 #import "ASDisplayNode.h"
 #import "ASSentinel.h"
 #import "ASThread.h"
+#import "ASLayout.h"
 
-BOOL ASDisplayNodeSubclassOverridesSelector(Class subclass, SEL selector);
-CGFloat ASDisplayNodeScreenScale();
 void ASDisplayNodePerformBlockOnMainThread(void (^block)());
 
 typedef NS_OPTIONS(NSUInteger, ASDisplayNodeMethodOverrides) {
@@ -51,7 +50,7 @@ typedef NS_OPTIONS(NSUInteger, ASDisplayNodeMethodOverrides) {
   // This is the desired contentsScale, not the scale at which the layer's contents should be displayed
   CGFloat _contentsScaleForDisplay;
 
-  CGSize _size;
+  ASLayout *_layout;
   CGSize _constrainedSize;
   UIEdgeInsets _hitTestSlop;
   NSMutableArray *_subnodes;