* Initial commit for ASStackLayoutSpec improvements
* Remove the lock in ASStackLayoutSpec and make the ASStackLayoutSpecStyle const
I think we don't need lock here as the style already has a lock while we set the value
* Add ASStackLayoutSpecItem that replaces layout specific items
* Prevent baseline pass if not needed
* Update comments
* [Layout API] Rename ASLayoutable to ASLayoutElement
* arg
* fix wrapperWithLayoutElement
* [Layout API] Rename ASLayoutable to ASLayoutElement
* arg
* address Michael's comments
* Remove ASLayoutValidation files that were deleted on master since this diff was created.
- ASLayoutable requires mutable properties that are used when attached to a stack layout.
- Thus, ASLayoutable objects (including ASDisplayNode) can be injected into stack layout directly.
- ASStackLayoutNodeChild no longer needed.
- Tests and Kitten sample updated.
- The code is forked from LayoutComponents in ComponentKit.
- Public interfaces are modified to be strictly Objective-C. As a result, users are not forced to switch to Objective-C++, the linker can happily compile and Swift fans can continue using the mighty ASDK.