layout_2020: Refactor PositioningContext to be simpler and smaller
layout_2020: Refactor PositioningContext to be simpler and smaller

Add a few helper methods which allow removing duplicate code in
PositioningContext. These methods will also be used to properly
implement hoisting in inline layout.

splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo