Use a map to make mon_at() 144% faster.

... when you have a giant horde of zombies (thanks to a 50x spawn rate).

Actually, the 144% speed improvement is for the portion of game::do_turn() from immediately after process_activities() through the end of the function (including the monmove() call). These changes brought that time from 2.92 seconds per turn down to 1.19 in my test savefile. The boost to mon_at() specifically must be even greater, since not all of do_turn() is spent in mon_at().