cleanup map::destroy_vehicle
port map::displace_water to use tripoints
port map::flammable_items_at and map::moppable_items_at to use tripoints
port map::has_nearby_fire and map::mop_spills to use tripoints
Make map::destroy use tripoints
ported several field related functions to use tripoints

map::step_in_field, map::mon_in_field, map::field_effect, and map::remove_field use tripoints exclusively