[Layout] Move [ASLayoutSpec children] from std::map to NSMutableArray (#2253)

* Initial commit to move [ASLayoutSpec children] from std::map to NSMutableArray

* Add NSFastEnumeration to ASLayoutable

* ASNullLayoutSpec is a Singleton now

* Move ASLayoutSpecPrivate in Private folder

* Move to NSArrayPointer and remove ASNullLayoutSpec

* Revert "Move to NSArrayPointer and remove ASNullLayoutSpec"

This reverts commit 9ab9cf7024b1f6e1984d84fe58af2b84e84cdf94.

* Move to childAtIndex: and setChild:atIndex:

AsyncDisplayKit/Private/ASLayoutSpecPrivate.h

@@ -0,0 +1,25 @@
+//
+//  ASLayoutSpecPrivate.h
+//  AsyncDisplayKit
+//
+//  Created by Michael Schneider on 9/15/16.
+//
+//  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 "ASInternalHelpers.h"
+#import "ASEnvironmentInternal.h"
+#import "ASThread.h"
+
+@interface ASLayoutSpec() {
+  ASDN::RecursiveMutex __instanceLock__;
+  ASEnvironmentState _environmentState;
+  ASLayoutableStyle *_style;
+  NSMutableArray *_childrenArray;
+}
+@end
+