Clean up #468:

- -layoutSpecThatFits: must return an ASLayoutSpec.
- Move ASDisplayNode's -measureWithSizeRange: redeclaration to ASDisplayNode.h.
- Rename ASStackLayoutChild.h to ASStackLayoutDefines.h.
- Rename ASStaticLayoutSpecDimension.h to ASRelativeSize.h.
- Don't import ASLayout.h in other headers to prevent circular inclusions.
- Explain use cases of ASLayout's initializers.
- Clean up ASInternalHelpers.h.

AsyncDisplayKit/Private/ASInternalHelpers.h

@@ -8,11 +8,9 @@
  *
  */
 
-#import <UIKit/UIKit.h>
+#include <CoreGraphics/CGBase.h>
 #import "ASBaseDefines.h"
 
-@class ASLayoutChild;
-
 ASDISPLAYNODE_EXTERN_C_BEGIN
 
 BOOL ASSubclassOverridesSelector(Class superclass, Class subclass, SEL selector);