[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/Layout/ASLayoutSpec+Subclasses.h

@@ -0,0 +1,64 @@
+//
+//  ASLayoutSpec+Subclasses.h
+//  AsyncDisplayKit
+//
+//  Created by Michael Schneider on 9/15/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ASLayoutSpec (Subclassing)
+
+/**
+ * Helper method for finalLayoutable support
+ *
+ * @warning If you are getting recursion crashes here after implementing finalLayoutable, make sure
+ * that you are setting isFinalLayoutable flag to YES. This must be one BEFORE adding a child
+ * to the new ASLayoutable.
+ *
+ * For example:
+ * - (id<ASLayoutable>)finalLayoutable
+ * {
+ *   ASInsetLayoutSpec *insetSpec = [[ASInsetLayoutSpec alloc] init];
+ *   insetSpec.insets = UIEdgeInsetsMake(10,10,10,10);
+ *   insetSpec.isFinalLayoutable = YES;
+ *   [insetSpec setChild:self];
+ *   return insetSpec;
+ * }
+ *
+ * @see finalLayoutable
+ */
+- (id<ASLayoutable>)layoutableToAddFromLayoutable:(id<ASLayoutable>)child;
+
+/**
+ * Adds a child with the given identifier to this layout spec.
+ *
+ * @param child A child to be added.
+ *
+ * @param index An index associated with the child.
+ *
+ * @discussion Every ASLayoutSpec must act on at least one child. The ASLayoutSpec base class takes the
+ * responsibility of holding on to the spec children. Some layout specs, like ASInsetLayoutSpec,
+ * only require a single child.
+ *
+ * For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
+ * a subclass can use the setChild method to set the "primary" child. It should then use this method
+ * to set any other required children. Ideally a subclass would hide this from the user, and use the
+ * setChild:forIndex: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChild
+ * property that behind the scenes is calling setChild:forIndex:.
+ */
+- (void)setChild:(id<ASLayoutable>)child atIndex:(NSUInteger)index;
+
+/**
+ * Returns the child added to this layout spec using the given index.
+ *
+ * @param index An identifier associated with the the child.
+ */
+- (nullable id<ASLayoutable>)childAtIndex:(NSUInteger)index;
+
+@end
+
+NS_ASSUME_NONNULL_END