[ASLayout] General Layout Cleanup (#2920)

* Remove duplicate import

* Move `ASLayoutController` related code out of CoreGraphics+ASConvenience

* Further cleanup

* Move private layout files to Privat/Layout

* Move ASLayoutElementStylePrivate into Private/Layout

* Further cleanup

* Move tvOS related files to tvOS folder

* Further cleanup

AsyncDisplayKit/Private/Layout/ASStackPositionedLayout.h

@@ -0,0 +1,23 @@
+//
+//  ASStackPositionedLayout.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.
+//
+
+#import "ASLayout.h"
+#import "ASDimension.h"
+#import "ASStackUnpositionedLayout.h"
+
+/** Represents a set of laid out and positioned stack layout children. */
+struct ASStackPositionedLayout {
+  const std::vector<ASStackLayoutSpecItem> items;
+
+  /** Given an unpositioned layout, computes the positions each child should be placed at. */
+  static ASStackPositionedLayout compute(const ASStackUnpositionedLayout &unpositionedLayout,
+                                         const ASStackLayoutSpecStyle &style,
+                                         const ASSizeRange &constrainedSize);
+};