Introduce type property to replace layout id check

AsyncDisplayKit/Layout/ASLayout.h

@@ -31,6 +31,11 @@ extern BOOL CGPointIsNull(CGPoint point);
  */
 @property (nonatomic, weak, readonly) id<ASLayoutable> layoutableObject;
 
+/**
+ * The type of ASLayoutable that created this layout
+ */
+@property (nonatomic, readonly) ASLayoutableType type;
+
 /**
  * Size of the current layout
  */