* Add tests for baseline alignments
* Merge baseline alignment algorithm to the main stack algorithm
- Baseline alignment is now part of the main stack algorithm.
- ASStackBaselinePositionedLayout is no longer needed and removed.
* All snapshot test cases of ASStackLayoutSpec are passing now
* Remove baselineRelativeArrangement
* Remove TODO
* Minor fixes in cross size determination step
1) Set the ascender/descender of an ASTextNode when the attributeString is set. Previously ascender/descender were only being computed in `setValuesFromLayoutable` and only when the attribute string was not nil. May make sense to remove the computation from `setValuesFromLayoutable` entirely.
2) Remove ability to allow different children of a stack spec to aling to different baselines. This wasn't working before and I'm not convinced it is possible to do properly/useful enough to invest the time.
3) Have all stack spec run `ASStackBaselinePositionedLayout::compute` to compute the stack's ascender and descender. Even if the stack isn't aligning its children to a baseline, the stack itself may be a child of another stack that IS aligning to a baseline.
