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/ASDisplayNodeInternal.h

@@ -17,7 +17,6 @@
 #import "ASDisplayNode.h"
 #import "ASSentinel.h"
 #import "ASThread.h"
-#import "ASLayout.h"
 
 BOOL ASDisplayNodeSubclassOverridesSelector(Class subclass, SEL selector);
 void ASDisplayNodePerformBlockOnMainThread(void (^block)());