[Layout] Add helper properties for setting sizes via CGSize and ASRelativeSize (#2324)

* Remove set layout style size helper

* Update API to set and get a CGSize / ASRelativeSize from an ASLayoutElementStyle

- Expose ASRelativeSize type
- Add new helper properties to set a size / relative size to ASLayoutElementStyle
- Don't expose the size in ASLayoutElementStyle anymore and move into ASLayoutElementStylePrivate

* Update examples

* Update comments for size helpers

AsyncDisplayKit/ASDisplayNode.mm

@@ -27,6 +27,7 @@
 #import "ASRunLoopQueue.h"
 #import "ASEnvironmentInternal.h"
 #import "ASDimension.h"
+#import "ASLayoutElementStylePrivate.h"
 
 #import "ASInternalHelpers.h"
 #import "ASLayout.h"
@@ -3478,8 +3479,7 @@ ASEnvironmentLayoutExtensibilityForwarding
   ASDN::MutexLocker l(__instanceLock__);
 
   // Deprecated preferredFrameSize just calls through to set width and height
-  _style.width = ASDimensionMake(preferredFrameSize.width);
-  _style.height = ASDimensionMake(preferredFrameSize.height);
+  _style.preferredSize = preferredFrameSize;
   [self invalidateCalculatedLayout];
 }