[Layout API] Rename ASStaticLayoutSpec to ASAbsoluteLayoutSpec (#2288)

* Rename static to absolute

* Fix examples, rename ASAbsoluteLayoutSpec class method

* rename +staticLayoutSpecWithChildren to +absoluteLayoutSpecWithChildren

* address Michael's comments

AsyncDisplayKit/Layout/ASAbsoluteLayoutSpec.h

@@ -0,0 +1,29 @@
+//
+//  ASAbsoluteLayoutSpec.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 <AsyncDisplayKit/ASLayoutSpec.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ * A layout spec that positions children at fixed positions.
+ * 
+ * Computes a size that is the union of all childrens' frames.
+ */
+@interface ASAbsoluteLayoutSpec : ASLayoutSpec
+
+/**
+ @param children Children to be positioned at fixed positions, each conforms to ASAbsoluteLayoutable
+ */
++ (instancetype)absoluteLayoutSpecWithChildren:(NSArray<id<ASAbsoluteLayoutable>> *)children;
+
+@end
+
+NS_ASSUME_NONNULL_END