The world contains rocks, pits, gold, and agents. Monsters are agents that act autonomously and kill any other agents in the same grid location. Pits are deadly.
Messages inform the user of any effects.
The top level function for sr-world. Iterates until no player is left alive. Returns true if the stimulus indicates that it is safe and possible to go left. Returns true if the stimulus indicates that it is safe and possible to go right. Returns true if the stimulus indicates that it is safe and possible to go backwards.