Add Layout nodes.

- The code is forked from LayoutComponents in ComponentKit.
- Public interfaces are modified to be strictly Objective-C. As a result, users are not forced to switch to Objective-C++, the linker can happily compile and Swift fans can continue using the mighty ASDK.

AsyncDisplayKit/Layout/ASLayout.h

@@ -0,0 +1,45 @@
+/*
+ *  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.
+ *
+ */
+
+#import <UIKit/UIKit.h>
+#import <AsyncDisplayKit/ASAssert.h>
+
+@class ASLayoutNode;
+
+/** Represents the computed size of a layout node, as well as the computed sizes and positions of its children. */
+@interface ASLayout : NSObject
+
+@property (nonatomic, readonly) ASLayoutNode *node;
+@property (nonatomic, readonly) CGSize size;
+/** 
+ * Each item is of type ASLayoutChild. 
+ */
+@property (nonatomic, readonly) NSArray *children;
+
++ (instancetype)newWithNode:(ASLayoutNode *)node size:(CGSize)size children:(NSArray *)children;
+
+/**
+ * Convenience that does not have any children.
+ */
++ (instancetype)newWithNode:(ASLayoutNode *)node size:(CGSize)size;
+
+@end
+
+@interface ASLayoutChild : NSObject
+
+@property (nonatomic, readonly) CGPoint position;
+@property (nonatomic, readonly) ASLayout *layout;
+
+/**
+ * Designated initializer
+ */
++ (instancetype)newWithPosition:(CGPoint)position layout:(ASLayout *)layout;
+
+@end