making undergound NPCs hog less CPU

NPCs no longer flail about looking for various shops.
Functionally, they still do the same thing. Just much faster.
Un-macro iterating across the skills vector.
Merge pull request #28 from zpmorgan/overmap-boundary-mapgen

fixing that problem where vehicles evaporate
Merge pull request #29 from zpmorgan/skill-rust

fixing skills, which were rusting almost every turn.
Merge pull request #30 from zpmorgan/master

bounds checking for veh_exists_at.
Merge pull request #31 from zpmorgan/master

map rendering optimization
Greatly reduce the transmission of scent across closed doors, windows, etc.
Fixed misplaced bracket in examine() code.