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/Private/ASDisplayNodeInternal.h

@@ -211,6 +211,9 @@ FOUNDATION_EXPORT NSString * const ASRenderingEngineDidDisplayNodesScheduledBefo
   
   NSString *_debugName;
 
+#pragma mark - ASDisplayNode (Debugging)
+  ASLayout *_unflattenedLayout;
+
 #if TIME_DISPLAYNODE_OPS
 @public
   NSTimeInterval _debugTimeToCreateView;