Small changes required by the coming layout debugger (#337)

* Small changes required by the layout debugger
- `ASDisplayNode` can be told to not flatten its layout immediately but later on. The unflattened layout is also stored in a separate property. It's needed for inspecting not only display nodes but also layout specs used to compute a layout tree.
- `ASLayout` can be told to always retain its sublayout elements. This is needed especially for layout specs since they are usually not retained after an ASLayout was computed.

* Update CHANGELOG

* Address comments

Source/ASDisplayNode.h

@@ -553,6 +553,30 @@ extern NSInteger const ASDefaultDrawingPriority;
  */
 @interface ASDisplayNode (Debugging) <ASDebugNameProvider>
 
+/**
+ * Set to YES to tell all ASDisplayNode instances to store their unflattened layouts.
+ *
+ * The layout can be accessed via `-unflattenedCalculatedLayout`.
+ *
+ * Flattened layouts use less memory and are faster to lookup. On the other hand, unflattened layouts are useful for debugging
+ * because they preserve original information.
+ */
++ (void)setShouldStoreUnflattenedLayouts:(BOOL)shouldStore;
+
+/**
+ * Whether or not ASDisplayNode instances should store their unflattened layouts. 
+ *
+ * The layout can be accessed via `-unflattenedCalculatedLayout`.
+ * 
+ * Flattened layouts use less memory and are faster to lookup. On the other hand, unflattened layouts are useful for debugging
+ * because they preserve original information.
+ *
+ * Defaults to NO.
+ */
++ (BOOL)shouldStoreUnflattenedLayouts;
+
+@property (nonatomic, strong, readonly, nullable) ASLayout *unflattenedCalculatedLayout;
+
 /**
  * @abstract Return a description of the node hierarchy.
  *