Add spacing properties

Summary:
Align C# implementation with YogaKit #322
Closes https://github.com/facebook/yoga/pull/327

Reviewed By: emilsjolander

Differential Revision: D4390687

Pulled By: splhack

fbshipit-source-id: 28c87a45898fcd958a422d5e254ead0ec00d3562

csharp/Facebook.Yoga/Spacing.cs

@@ -9,6 +9,7 @@
 
 namespace Facebook.Yoga
 {
+    [System.Obsolete]
     public class Spacing
     {
         public YogaValue? Top;