This PR moves highway areas below all linear road types (resolves gravitystorm#3281).

In particular, this change has the following effects:

* Render highway areas below tunnels (resolves gravitystorm#529). This prevents the current
  situation where tunnels are invisible if there happens to be a highway area
  above them.
* Render highway areas below line/area barriers, ferry routes, tourism
  boundaries, cliffs, landuse-overlay, and turning circles. I think these
  changes are mainly neutral or positive.
* Render highway areas below line/area barriers. This is probably the most
  controversial aspect (but necessary for the other changes). See screenshot
  below.

* This PR is a necessary condition for merging the roads-casing and roads-fill
  layers (part of gravitystorm#2046), which would greatly simplify our code.
* This PR is a necessary condition for rendering buildings above highway area
  (gravitystorm#688).

* Promoting linear highways over areas makes life easier for other data
  consumers, that already tend to have poor support for highway areas. For
  example:
  * Transport and wikipedia do not render road areas
  * Humanitarian, bicycle, mapbox, streets, OSM bright renders linear roads on
  top of areas, like in this proposal