Add helper function for bulk updates.

Summary:
Currently, our configuration of Views looks like this:

```objc
UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
view.yoga.isEnabled = YES;
view.yoga.height = 50;
view.yoga.width = 50;
```

Every time that we access `view.yoga` we have to access the associated object on `UIView` to get the `YGLayout`. This adds an extra `objc_msgSend` which increases binary size, and is slight perf impact.

This diff creates a way to modify the `YGLayout` with only a single `objc_msgSend`. Here's the new syntax:

```objc
UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
[view configureLayoutWithBlock:^void(YGLayout *layout){
  layout.isEnabled = YES
  layout.height = 50;
  layout.width = 50;
}];
```

Here's the Swift version:
```swift
let view = UIView(frame: .zero)
view.configureLayout { (layout) in
  layout.isEnabled = true
  layout.height = 50
  layout.width = 50
}
```
Closes https://github.com/facebook/yoga/pull/393

Reviewed By: emilsjolander

Differential Revision: D4550382

Pulled By: dshahidehpour

fbshipit-source-id: 76d797d1e0de8e5dc767e02180a7fc440a70212e

YogaKit/Tests/YogaKitTests.m

@@ -18,6 +18,25 @@
 
 @implementation YogaKitTests
 
+- (void)testConfigureLayoutIsNoOpWithNilBlock
+{
+  UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
+  XCTAssertNoThrow([view configureLayoutWithBlock:nil]);
+}
+
+- (void)testConfigureLayoutBlockWorksWithValidBlock
+{
+  UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
+  [view configureLayoutWithBlock:^(YGLayout *layout){
+    XCTAssertNotNil(layout);
+    layout.isEnabled = YES;
+    layout.width = 25;
+  }];
+
+  XCTAssertTrue(view.yoga.isEnabled);
+  XCTAssertEqual(view.yoga.width, 25);
+}
+
 - (void)testNodesAreDeallocedWithSingleView
 {
   __weak YGLayout *layoutRef = nil;