rename mapgen: { type: (builtin|lua|json), ..} from type to method, due to clash with convention
mapgen_json: 'set' array of abstracted functions which are one of:
"terrain", "furniture", "trap", "radiation" and correspond to map::set_.. functions,
with optional randomization of coordinates, chance (one_in ...), and repeat values
mapgen: json load via 'type' method as well as a part of overmap_terrain
fix weight calculation
cleanup / fixes etc
disable horrifically messy examples
field_test cpp function: nuke
move massive todo blob of commented code