Overhaul header files / includes to eliminate circular references that Xcode 7 is angry about.

AsyncDisplayKit/Private/ASDisplayNodeInternal.h

@@ -11,7 +11,6 @@
 // These methods must never be called or overridden by other classes.
 //
 
-#import "_ASDisplayLayer.h"
 #import "_AS-objc-internal.h"
 #import "ASDisplayNodeExtraIvars.h"
 #import "ASDisplayNode.h"
@@ -19,6 +18,9 @@
 #import "ASThread.h"
 #import "ASLayoutOptions.h"
 
+@protocol _ASDisplayLayerDelegate;
+@class _ASDisplayLayer;
+
 BOOL ASDisplayNodeSubclassOverridesSelector(Class subclass, SEL selector);
 void ASDisplayNodeRespectThreadAffinityOfNode(ASDisplayNode *node, void (^block)());
 
@@ -39,7 +41,7 @@ typedef NS_OPTIONS(NSUInteger, ASDisplayNodeMethodOverrides)
 
 #define TIME_DISPLAYNODE_OPS (DEBUG || PROFILE)
 
-@interface ASDisplayNode () <_ASDisplayLayerDelegate>
+@interface ASDisplayNode ()
 {
 @protected
   // Protects access to _view, _layer, _pendingViewState, _subnodes, _supernode, and other properties which are accessed from multiple threads.