- Refactor naming of ASEnvironmentCollection to ASEnvironmentState
- Remove struct pointers
- Move ASEnvironmentStatePropagation to a enum class
- Move merge functions to pure functions
- Move ASLayoutOptionsForwarding and ASLayoutableExtensibility into ASLayoutSpec and ASDisplayNode
- Remove ASLayoutableSetValuesForLayoutable and move into explicit classes (ASDisplayNode, ASTextNode)
- -layoutSpecThatFits: must return an ASLayoutSpec.
- Move ASDisplayNode's -measureWithSizeRange: redeclaration to ASDisplayNode.h.
- Rename ASStackLayoutChild.h to ASStackLayoutDefines.h.
- Rename ASStaticLayoutSpecDimension.h to ASRelativeSize.h.
- Don't import ASLayout.h in other headers to prevent circular inclusions.
- Explain use cases of ASLayout's initializers.
- Clean up ASInternalHelpers.h.