* Implement tests for the layout flattening process
* Refactor the flattening algorithm
- Remove flattened flag
- No more self check
- Stop traversing a layout tree branch when hits a displaynode node.
- Reuse as many existing ASLayout objects as possible
* Update changelog
* Ceil position values before comparing
* Explain why sublayout elements must be retained
* [Yoga] Implement ASYogaLayoutSpec, an experimental alternative to full-tree integration.
This approach allows us to avoid any ASDisplayNode.mm integration points.
However, it is not yet proven to be possible to achieve correctness with this approach.
The entry point (to start calculating), and the measurement function inputs, lack
the full expressiveness of ASSizeRange; we need to make sure that workarounds like
using style.minSize are successful in simulating the behavior of a full Yoga tree.
* [Yoga] Fix file comments, move towards <ASLayoutElement> support.
* [Yoga] Important fix for simplified, non-contiguous Yoga integration.
* [Yoga] Complete implementation of manual memory management (__bridge_transfer, YGNodeFree)
