Fix callbacks on AOT

Summary:
When using AOT mode on Mono one can't use instance methods for callbacks, the compiler needs the MonoPInvokeCallback attribute on a static method to know how to get back to the managed world.
This worked fine without the change in JIT mode.

I not sure if we should use a __MONO__ flag for this stuff as this could be needed for other usage that not only on iOS.

~~The adicional change is that one should as good practice call Dispose on the YogaNode when using callbacks to make sure we remove the handler from our dictionary, i was trying to write a test for this.. but i saw TestMeasureFuncWithDestructor was already there, but it doesn't seem correct to me, can you check splhack.~~
Closes https://github.com/facebook/yoga/pull/386

Reviewed By: emilsjolander

Differential Revision: D4546030

Pulled By: splhack

fbshipit-source-id: 121d44a2ecd14d754b9614493db6ac15fca02da2

csharp/Facebook.Yoga/Native.cs

@@ -111,6 +111,13 @@ namespace Facebook.Yoga
         [return: MarshalAs(UnmanagedType.I1)]
         public static extern bool YGNodeGetHasNewLayout(YGNodeHandle node);
 
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGNodeSetContext(YGNodeHandle node, IntPtr context);
+
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern IntPtr YGNodeGetContext(IntPtr node);
+
+
         #endregion
 
         #region YG_NODE_STYLE_PROPERTY