* 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
- 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.