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/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": {