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.xcodeproj/xcshareddata/xcschemes/Sample.xcscheme

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