[Layout] Add helper properties for setting sizes via CGSize and ASRelativeSize (#2324)

* Remove set layout style size helper

* Update API to set and get a CGSize / ASRelativeSize from an ASLayoutElementStyle

- Expose ASRelativeSize type
- Add new helper properties to set a size / relative size to ASLayoutElementStyle
- Don't expose the size in ASLayoutElementStyle anymore and move into ASLayoutElementStylePrivate

* Update examples

* Update comments for size helpers

AsyncDisplayKit/Layout/ASLayoutElementStylePrivate.h

@@ -0,0 +1,27 @@
+//
+//  ASLayoutElementStylePrivate.h
+//  AsyncDisplayKit
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#pragma once
+
+@interface ASLayoutElementStyle ()
+
+/**
+ * @abstract The object that acts as the delegate of the style.
+ *
+ * @discussion The delegate must adopt the ASLayoutElementStyleDelegate protocol. The delegate is not retained.
+ */
+@property (nullable, nonatomic, weak) id<ASLayoutElementStyleDelegate> delegate;
+
+/**
+ * @abstract A size constraint that should apply to this ASLayoutElement.
+ */
+@property (nonatomic, assign, readonly) ASLayoutElementSize size;
+
+@end