Switched to using SafeHandle for native code interrupt

Summary:
This PR fixes issue #295

- Created a new internal YGNodeHandle class extending SafeHandle.
- YogaNode now stores a reference to YGNodeHandle.
- Removed finalizer from YogaNode.
- Pulling in System.Runtime.Handles 4.3.0.
Closes https://github.com/facebook/yoga/pull/297

Reviewed By: emilsjolander

Differential Revision: D4365462

Pulled By: splhack

fbshipit-source-id: 73293b105962cbedb71f4e3d8d1244251db1ea79

csharp/Facebook.Yoga/YogaNode.cs

@@ -17,7 +17,7 @@ namespace Facebook.Yoga
 {
     public partial class YogaNode : IEnumerable<YogaNode>
     {
-        private IntPtr _ygNode;
+        private Native.YGNodeHandle _ygNode;
         private WeakReference _parent;
         private List<YogaNode> _children;
         private MeasureFunction _measureFunction;
@@ -29,17 +29,12 @@ namespace Facebook.Yoga
             YogaLogger.Initialize();
 
             _ygNode = Native.YGNodeNew();
-            if (_ygNode == IntPtr.Zero)
+            if (_ygNode.IsInvalid)
             {
                 throw new InvalidOperationException("Failed to allocate native memory");
             }
         }
 
-        ~YogaNode()
-        {
-            Native.YGNodeFree(_ygNode);
-        }
-
         public void Reset()
         {
             _measureFunction = null;