Refactor submap::trp to use public getters and setters instead. [CR]

I'm honestly not sure whether this is worthwhile. The getters/setters might be good to have, but don't know whether the has_traps() flag is worth it.

Note that the concern here is having 11 times more submaps loaded, which could increase the cost of map::shift quite a bit without optimizations like this. Most submaps will be rock/open air, so no traps need to be cleared there.