Switched to using SafeHandle for native code interrupt.

- 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.

csharp/Facebook.Yoga/project.json

@@ -3,7 +3,8 @@
   "version": "3.0.0-*",
 
   "dependencies": {
-    "NETStandard.Library": "1.6.0"
+    "NETStandard.Library": "1.6.0",
+    "System.Runtime.Handles": "4.3.0"
   },
 
   "frameworks": {