Adds layout controller for collection views. This new layout controller leverages UICollectionViewLayout's layoutAttributesForElementsInRect in order to calculate index paths for items in AsyncDisplayKit ranges. Flow layout is still a requirement in order to test the waters.

AsyncDisplayKit/Details/ASCollectionViewLayoutController.h

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+/* Copyright (c) 2015-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 <AsyncDisplayKit/ASAbstractLayoutController.h>
+#import <AsyncDisplayKit/ASBaseDefines.h>
+
+@class ASCollectionView;
+
+@interface ASCollectionViewLayoutController : ASAbstractLayoutController
+
+- (instancetype)initWithCollectionView:(ASCollectionView *)collectionView;
+
+@end