This function returns which room is reached by a given exit from this room. The string argument specifies which exit is meant, and should be one of the strings 'north', 'south', 'east', 'west', 'up', and 'down'. If no room is given (that is, the second form of the argument list is used) then the room is taken to be the one containing the entity with the executing MUDL script.
If the given exit doesn't exist, or doesn't lead to a room, this function returns null.