[ASCollectionNode] Add support for using UICollectionViewCells

[hannahmbanana]

Paired programming with @appleguy.

This method offers compatibility with synchronous, standard UICollectionViewCell objects. These cells will not have the performance benefits of ASCellNodes (like preloading, async layout, and async drawing) - even when mixed within the same ASCollectionNode.

In order to use this method, you must:

1. Implement it on your ASCollectionDataSource object.
2. Call registerCellClass: on the collectionNode.view (in viewDidLoad, or register an onDidLoad: block).
3. Return nil from the nodeBlockForItem...: or nodeForItem...: method. NOTE: it is an error to return nil from within a nodeBlock, if you have returned a nodeBlock object.
4. Lastly, you must implement a method to provide the size for the cell. There are two ways this is done:
   4a. UICollectionViewFlowLayout (incl. ASPagerNode). Implement collectionNode:constrainedSizeForItemAtIndexPath:.
   4b. Custom collection layouts. Set .view.layoutInspector and have it implement collectionView:constrainedSizeForNodeAtIndexPath:.

For an example of using this method with all steps above (including a custom layout, 4b.), see the app in examples/CustomCollectionView and enable kShowUICollectionViewCells = YES (example project included in this PR).

[hannahmbanana]

getting rid of this because we're dropping iOS 7 support

[hannahmbanana]

Is there a better way to do this?

[hannahmbanana]

cc @KieranLafferty

[Adlai-Holler]

We should revisit this after/in light of #2942 which has a lot of these same hooks.

[QuynhNguyen]

Just wondering, what are we getting out of this? From the description, it seems like using ASCellNode over UICollectionViewCell is a no brainer.

[hannahmbanana]

@QuynhNguyen - great question!

ASCellNode is always preferred. However, for code bases that have hundreds or more custom UICollectionViewCells subclasses, this feature will allow them to port to ASDK more easily, and then slowly convert their backlog of cells to be ASCellNode subclasses.

This change allows mixing both those un-ported UICollectionViewCells and ASCellNodes without a performance penalty for the UIKitCells. In addition, it still provides of ASCellNode, including automatic preload, asynchronous measurement and asynchronous rendering. Because of this difference in performance, app developers will clearly see the value of porting cells to ASCellNode and be able to do so over time.

That said, I think @Adlai-Holler implemented this feature in #2942 so we may not need this PR anymore.

[Adlai-Holler]

I'm closing this PR – supporting this use case will be much different in light of #2942 so let's take a fresh stab at it. If you disagree and want to fix forward from this PR, feel free to reopen.