Mapping changes : generic code

Notice the new two functions get_wall_state and set_wall_state, and the
new code for update_map !

Do you want to get the wall state for north of (x,y) ?
Easy, just write : get_wall_state(_map[x][y],NORTH);
Then you can compare the return value to NO_WALL, IS_WALL or UNKNOWN !

Example for set_wall_state :
U8 data = _map[x][y];
set_wall_state(&data, WEST, IS_WALL);
Remove unused define in movement