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/ASStackUnpositionedLayout.h

@@ -0,0 +1,36 @@
+/*
+ *  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 <vector>
+
+#import "ASLayout.h"
+#import "ASStackLayoutNode.h"
+
+struct ASStackUnpositionedItem {
+  /** The original source child. */
+  ASStackLayoutNodeChild *child;
+  /** The proposed layout. */
+  ASLayout *layout;
+};
+
+/** Represents a set of stack layout children that have their final layout computed, but are not yet positioned. */
+struct ASStackUnpositionedLayout {
+  /** A set of proposed child layouts, not yet positioned. */
+  const std::vector<ASStackUnpositionedItem> items;
+  /** The total size of the children in the stack dimension, including all spacing. */
+  const CGFloat stackDimensionSum;
+  /** The amount by which stackDimensionSum violates constraints. If positive, less than min; negative, greater than max. */
+  const CGFloat violation;
+
+  /** Given a set of children, computes the unpositioned layouts for those children. */
+  static ASStackUnpositionedLayout compute(const std::vector<ASStackLayoutNodeChild *> &children,
+                                           const ASStackLayoutNodeStyle &style,
+                                           const ASSizeRange &sizeRange);
+};