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/Private/ASStackPositionedLayout.h

@@ -0,0 +1,25 @@
+/*
+ *  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 "ASLayout.h"
+#import "ASDimension.h"
+#import "ASStackLayoutNode.h"
+#import "ASStackUnpositionedLayout.h"
+
+/** Represents a set of laid out and positioned stack layout children. */
+struct ASStackPositionedLayout {
+  const std::vector<ASLayoutChild *> children;
+  const CGFloat crossSize;
+
+  /** Given an unpositioned layout, computes the positions each child should be placed at. */
+  static ASStackPositionedLayout compute(const ASStackUnpositionedLayout &unpositionedLayout,
+                                         const ASStackLayoutNodeStyle &style,
+                                         const ASSizeRange &constrainedSize);
+};