[Yoga] Rewrite YOGA_TREE_CONTIGUOUS mode with improved behavior and cleaner integration (#343)

* [Yoga] Rewrite YOGA_TREE_CONTIGUOUS mode with support for mixing with ASLayoutSpec.

After experimentation with the ASYogaLayoutSpec (or non-contiguous) approach to
integrating Yoga, test results and feedback from the authors of Yoga have shown
that this approach can't be made completely correct,

There are issues with some of the features required to represent Web-style
flexbox; in particular: padding, margins, and border handling have varience.

This diff is a first step towards a truly correct and elegant implementation of
Yoga integration with Texture. In addition to reducing the footprint of
the integration, which is an explicit goal of work at this stage, these changes
already support improved behavior - including mixing between ASLayoutSpecs
even as subnodes of Yoga layout-driven nodes, in addition to above them. Yoga
may be used for any set of nodes.

Because Yoga usage is limited at this time, it's safe to merge this diff and
further improvements will be refinements in this direction.

* [ASDKgram] Add Yoga layout implementation for PhotoCellNode.

* [Yoga] Final fixes for the upgraded implementation of the Contiguous layout mode.

* [Yoga] Add CHANGELOG.md entry and fix for Yoga rounding to screen scale.

* [Yoga] Minor cleanup to remove old comments and generalize utility methods.

Source/Private/ASDisplayNodeInternal.h

@@ -58,6 +58,7 @@ typedef NS_OPTIONS(NSUInteger, ASDisplayNodeMethodOverrides)
 typedef NS_OPTIONS(uint_least32_t, ASDisplayNodeAtomicFlags)
 {
   Synchronous = 1 << 0,
+  YogaLayoutInProgress = 1 << 1,
 };
 
 #define checkFlag(flag) ((_atomicFlags.load() & flag) != 0)
@@ -204,7 +205,6 @@ FOUNDATION_EXPORT NSString * const ASRenderingEngineDidDisplayNodesScheduledBefo
   NSMutableArray<ASDisplayNode *> *_yogaChildren;
 #endif
 #if YOGA_TREE_CONTIGUOUS
-  YGNodeRef _yogaNode;
   __weak ASDisplayNode *_yogaParent;
   ASLayout *_yogaCalculatedLayout;
 #endif