Fixes to baseline stack alignment

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.

AsyncDisplayKit/Private/ASStackLayoutSpecUtilities.h

@@ -63,10 +63,6 @@ inline ASStackLayoutAlignItems alignment(ASStackLayoutAlignSelf childAlignment,
       return ASStackLayoutAlignItemsStart;
     case ASStackLayoutAlignSelfStretch:
       return ASStackLayoutAlignItemsStretch;
-    case ASStackLayoutAlignSelfBaselineFirst:
-      return ASStackLayoutAlignItemsBaselineFirst;
-    case ASStackLayoutAlignSelfBaselineLast:
-      return ASStackLayoutAlignItemsBaselineLast;
     case ASStackLayoutAlignSelfAuto:
     default:
       return stackAlignment;