Initial attempt at implementing Display Traits

Initial attempt to get display traits working with ASEnvironment.

To get proper ASDisplayTraits support, you must use an ASViewController. The ASViewController implements UITraitCollection-related methods (`traitCollectionDidChange:`, `willTransitionToTraitCollection:withTransitionCoordinator:`, viewWillTransitionToSize:withTransitionCoordinator`) to update the internal ASDisplayTraits and propagate them to subnodes.

ASTableNode and ASCollectionNode don't actually have their cells as subnodes, so a little bit of trickery is involved (on `setEnvironment:` the table/collection node gets its data controllers completedNodes and propagates the new traits. see `ASDisplayTraitsCollectionTableSetEnvironmentState`). The data controller also passes the current display traits when creating new cells.

ASViewController also supports the ability to return a custom set of display traits. So if you have a modal dialog that should always be told it is in a compact size class, you can set the override block before displaying the VC.

A new example, called Display Traits, has been added. It shows how display traits can be used in a ASViewController with a normal ASDisplayNode as its root, as well as in ASViewControllers hosting table nodes and collection nodes. There is also an example of overriding the default display traits of a VC.

Please provide feedback!

examples/DisplayTraits/Sample/CollectionViewController.m

@@ -0,0 +1,72 @@
+/* This file provided by Facebook is for non-commercial testing and evaluation
+ * purposes only.  Facebook reserves all rights not expressly granted.
+ *
+ * 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.
+ */
+
+#import "CollectionViewController.h"
+#import "KittenNode.h"
+
+@interface CollectionViewController () <ASCollectionDelegate, ASCollectionDataSource>
+@property (nonatomic, strong) ASCollectionNode *collectionNode;
+@end
+
+@implementation CollectionViewController
+
+- (instancetype)init
+{
+  UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
+  layout.minimumLineSpacing = 10;
+  layout.minimumInteritemSpacing = 10;
+  
+  ASCollectionNode *collectionNode = [[ASCollectionNode alloc] initWithCollectionViewLayout:layout];
+  
+  if (!(self = [super initWithNode:collectionNode]))
+    return nil;
+  
+  self.title = @"Collection Node";
+  _collectionNode = collectionNode;
+  collectionNode.dataSource = self;
+  collectionNode.delegate = self;
+  return self;
+}
+
+- (void)viewDidLoad
+{
+  [super viewDidLoad];
+  self.collectionNode.view.contentInset = UIEdgeInsetsMake(20, 10, CGRectGetHeight(self.tabBarController.tabBar.frame), 10);
+}
+
+#pragma mark - ASCollectionDataSource
+
+- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
+{
+  return 50;
+}
+
+- (ASCellNode *)collectionView:(ASCollectionView *)collectionView nodeForItemAtIndexPath:(NSIndexPath *)indexPath
+{
+  KittenNode *cell = [[KittenNode alloc] init];
+  cell.textNode.maximumNumberOfLines = 3;
+  cell.imageTappedBlock = ^{
+    [KittenNode defaultImageTappedAction:self];
+  };
+  return cell;
+}
+
+- (ASSizeRange)collectionView:(ASCollectionView *)collectionView constrainedSizeForNodeAtIndexPath:(NSIndexPath *)indexPath
+{
+  ASDisplayTraits displayTraits = self.collectionNode.environmentState.displayTraits;
+  
+  if (displayTraits.horizontalSizeClass == UIUserInterfaceSizeClassRegular) {
+    return ASSizeRangeMake(CGSizeMake(200, 120), CGSizeMake(200, 120));
+  }
+  return ASSizeRangeMake(CGSizeMake(132, 180), CGSizeMake(132, 180));
+}
+
+@end