Basic implementation of transitioning API for layout specs

AsyncDisplayKit/ASDisplayNode.h

@@ -571,6 +571,17 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
+@interface ASDisplayNode (Transitioning)
+
+/**
+ @abstract Invalidates the current layout and begins a relayout of the node to the new layout returned in `calculateLayoutThatFits:`.
+
+ @discussion Animation is optional, but will still proceed through the `transitionLayout` methods with `isAnimated == NO`.
+ */
+- (void)transitionLayoutWithAnimation:(BOOL)animated;
+
+@end
+
 
 /**
  * Convenience methods for debugging.