Support IL2CPP and fix stale handle

Summary:
- Unity IL2CPP (ENABLE_IL2CPP) requires the same code path with AOT compile.
- Clean up unneeded code in YGConfigHandle.
- YogaNode.Reset makes YGNodeHandle stale. Unmanaged side was reset by YGNodeReset, but YGNodeHandle keeps to retain the old managed handle. Release it.
Closes https://github.com/facebook/yoga/pull/491

Reviewed By: astreet

Differential Revision: D4765683

Pulled By: splhack

fbshipit-source-id: 83bfe19feb0e6ec45dc504e42b0c6c34e10af8e2

csharp/Facebook.Yoga/YogaLogger.cs

@@ -13,6 +13,9 @@ using System.Runtime.InteropServices;
 #if __IOS__
 using ObjCRuntime;
 #endif
+#if ENABLE_IL2CPP
+using AOT;
+#endif
 
 namespace Facebook.Yoga
 {
@@ -26,7 +29,7 @@ namespace Facebook.Yoga
 
         public static Func Logger = null;
 
-#if (UNITY_IOS && !UNITY_EDITOR) || __IOS__
+#if (UNITY_IOS && !UNITY_EDITOR) || ENABLE_IL2CPP || __IOS__
         [MonoPInvokeCallback(typeof(Func))]
 #endif
         public static void LoggerInternal(YogaLogLevel level, string message)