Introduce ASCollectionGalleryLayoutDelegate (#76)

* Implement ASCollectionGalleryLayoutDelegate
- It arranges items of the same size into a multi-line stack (say photo gallery or pager). It takes advantage of the fact that its items always have a fixed size to measure as few items as possible while still being able to track their positions at all time. This helps reduce startup/reloadData time, as well as memory footprint.
- It then uses a measure range, which also works as a allocate range, to figure out which items to measure ahead of time. And it guarantees that each item is scheduled to measure only once.
- Lastly, ASCollectionLayoutDelegate has some new methods that allow delegates to hook up and stay ahead of layout attributes requests from the backing view. ASCollectionGalleryLayoutDelegate for example uses these methods to ensure elements that have their layout attributes requested are always ready for consumption, and to measure more elements in the background.

* Handle items that span multiple pages and other improvements in gallery delegate

* Minor fixes

* Fix failing tests

* Fix custom collection example

* Implement missing method in gallery layout delegate

* Fix warnings

* Some improvements
- Collection layout delegates must have a crollable directions property.
- Simplify gallery delegate by not storing unmeasured attributes since calling measure on already measured elements should be cache hits and super fast.
- Abstact some code in gallery delegate to ASCollectionLayoutState+Private and _ASCollectionGalleryLayoutItem.
- Other improvements in gallery delegate

* Fix file licenses

* Move measure range logic to ASCollectionLayout

* Track unmeasured elements

* Remove pending layout in ASCollectionLayout

* Get back pending layout because the timing to latch new data is not ideal

* Add ASCollectionLayoutCache

* Fix file licenses

* Fix xcodeproj

* Add async collection layout to examples/ASCollectionView

* Measure method in ASCollectionLayout to be a class method

* Encourage more immutable states
- Make -calculateLayoutWithContext: to be class methods in ASDataControllerLayoutDelegate and ASCollectionLayoutDelegate.
- Add layout delegate class and layout cache to ASCollectionLayoutContext+Private, to be use by ASCollectionLayout only.
- ASDataController no longer allocates all nodes but lets ASCollectionLayout determine.
- Add scrollableDirections to the layout context since it's often needed by the layout pass. Otherwise users have to wrap it in an info object.
- Update built-in layout delegates and CustomCollectionView example.
- Publish ASHashing. It might be helpful for clients that implement custom collection info objects.

* Remove additionalInfo property in ASCollectionLayoutState

* ASCollectionLayoutState to correctly filter unmeasured elements

* Add ASHashing to umbrella header

* Fix file licenses

* Add ASDispatchAsync and use it in ASCollectionLayout

* Improve code comment in ASCollectionLayoutState

examples/ASCollectionView/Sample/ViewController.m

@@ -1,18 +1,18 @@
 //
 //  ViewController.m
-//  Sample
+//  Texture
 //
 //  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.
+//  LICENSE file in the /ASDK-Licenses directory of this source tree. An additional
+//  grant of patent rights can be found in the PATENTS file in the same directory.
 //
-//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
-//  FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
-//  ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
-//  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+//  Modifications to this file made after 4/13/2017 are: Copyright (c) 2017-present,
+//  Pinterest, Inc.  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
 //
 
 #import "ViewController.h"
@@ -21,6 +21,8 @@
 #import "SupplementaryNode.h"
 #import "ItemNode.h"
 
+#define ASYNC_COLLECTION_LAYOUT 0
+
 @interface ViewController () <ASCollectionDataSource, ASCollectionDelegateFlowLayout>
 
 @property (nonatomic, strong) ASCollectionNode *collectionNode;
@@ -43,8 +45,18 @@
 - (void)viewDidLoad
 {
   [super viewDidLoad];
+
+#if ASYNC_COLLECTION_LAYOUT
+  id<ASCollectionLayoutDelegate> layoutDelegate = [[ASCollectionGalleryLayoutDelegate alloc] initWithScrollableDirections:ASScrollDirectionVerticalDirections
+                                                                                                                 itemSize:CGSizeMake(180, 90)];
+  self.collectionNode = [[ASCollectionNode alloc] initWithLayoutDelegate:layoutDelegate layoutFacilitator:nil];
+#else
+  UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
+  layout.headerReferenceSize = CGSizeMake(50.0, 50.0);
+  layout.footerReferenceSize = CGSizeMake(50.0, 50.0);
+  self.collectionNode = [[ASCollectionNode alloc] initWithFrame:self.view.bounds collectionViewLayout:layout];
+#endif
   
-  self.collectionNode = [[ASCollectionNode alloc] initWithLayoutDelegate:[[ASCollectionFlowLayoutDelegate alloc] init] layoutFacilitator:nil];
   self.collectionNode.dataSource = self;
   self.collectionNode.delegate = self;