Add YogaKit Shared and iOS version

Summary:
Using shared code for reuse in other platforms based on iOS native implementation.

Adds YogaKit sample.

Adds YogaKit tests (same as objc).

```
YogaKitTest : 80 ms
Facebook.YogaKit.iOS.Tests.exe : 81 ms
Tests run: 11 Passed: 8 Inconclusive: 0 Failed: 3 Ignored: 1
```

Since we don't have extension properties we need to go with a extension method to get access to the YogaLayout .

I m also not sure this is leak free yet, would love some help with testing and feedback about view/node lifecycle
Closes https://github.com/facebook/yoga/pull/336

Reviewed By: splhack

Differential Revision: D4415027

Pulled By: emilsjolander

fbshipit-source-id: c88328212426c3200e6f0c48cda594cd2c432065

csharp/iOS/Facebook.YogaKit.iOS/YogaLayout.cs

@@ -0,0 +1,52 @@
+using System.Collections.Generic;
+using CoreGraphics;
+using Facebook.Yoga;
+using Foundation;
+using UIKit;
+
+namespace Facebook.YogaKit
+{
+	public partial class YogaLayout : NSObject
+	{
+		static IEnumerable<UIView> GetChildren(UIView view) => view.Subviews;
+
+		static void MeasureNativeView(UIView view, float constrainedWidth, float constrainedHeight, out float sizeThatFitsWidth, out float sizeThatFitsHeight)
+		{
+			var sizeThatFits = view.SizeThatFits(new CGSize(constrainedWidth, constrainedHeight));
+			sizeThatFitsWidth = (float)sizeThatFits.Width;
+			sizeThatFitsHeight = (float)sizeThatFits.Height;
+		}
+
+		static void GetWidthHeightOfNativeView(UIView view, out float width, out float height)
+		{
+			width = (float)view.Bounds.Width;
+			height = (float)view.Bounds.Height;
+		}
+
+		static float NativePixelScale => (float)UIScreen.MainScreen.Scale;
+
+
+		static void ApplyLayoutToNativeView(UIView view, YogaNode node)
+		{
+			var topLeft = new CGPoint(node.LayoutX, node.LayoutY);
+			var bottomRight = new CGPoint(topLeft.X + node.LayoutWidth, topLeft.Y + node.LayoutHeight);
+			view.Frame = new CGRect(RoundPixelValue((float)topLeft.X), RoundPixelValue((float)topLeft.Y), RoundPixelValue((float)bottomRight.X) - RoundPixelValue((float)topLeft.X), RoundPixelValue((float)bottomRight.Y) - RoundPixelValue((float)topLeft.Y));
+		}
+
+		bool _disposed;
+		protected override void Dispose(bool disposing)
+		{
+			if (disposing && !_disposed)
+			{
+				_disposed = true;
+				if (YogaKit.Bridges.ContainsKey(_node))
+				{
+					YogaKit.Bridges.Remove(_node);
+				}
+				_node = null;
+				_viewRef = null;
+			}
+			base.Dispose(disposing);
+		}
+	}
+}