[RFC / testing needed] Completely replace the oter_id enum with new overmap_tiles.json.

overmap terrain type ids are now strings.

overmap terrain types are now specified via json.

also:
* north/east/south/west rotations of houses and shops are now handled automatically.
* whether an overmap terrain type causes adjacent roads to have sidewalks, and whether roads can be constructed thru the terrain type are now bool options.
* overmap terrain types that are drawn using line-drawing characters now have their symbols implanted automatically.
* map symbols can be specified per-rotation (e.g. house), or once for all rotations (e.g. subway station).
* huge chunks of stuff were restyled because i touched it all.
* a bunch of other minor stuff was probably fixed.
* many mapgen-related things are potentially broken (although my tests look okay so far).
* savegame version was incremented from 11 to 12.

if you want to test this, back up your save first!

particularly looking for reports of missing or corrupted buildings or map areas.