Clean up #468:

- -layoutSpecThatFits: must return an ASLayoutSpec.
- Move ASDisplayNode's -measureWithSizeRange: redeclaration to ASDisplayNode.h.
- Rename ASStackLayoutChild.h to ASStackLayoutDefines.h.
- Rename ASStaticLayoutSpecDimension.h to ASRelativeSize.h.
- Don't import ASLayout.h in other headers to prevent circular inclusions.
- Explain use cases of ASLayout's initializers.
- Clean up ASInternalHelpers.h.

AsyncDisplayKit/Layout/ASStackLayoutDefines.h

@@ -0,0 +1,22 @@
+/*
+ *  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.
+ *
+ */
+
+/**
+ Each child may override their parent stack's cross axis alignment.
+ @see ASStackLayoutAlignItems
+ */
+typedef NS_ENUM(NSUInteger, ASStackLayoutAlignSelf) {
+  /** Inherit alignment value from containing stack. */
+  ASStackLayoutAlignSelfAuto,
+  ASStackLayoutAlignSelfStart,
+  ASStackLayoutAlignSelfEnd,
+  ASStackLayoutAlignSelfCenter,
+  ASStackLayoutAlignSelfStretch,
+};