Implementation of the path command for robots.

Design considerations

One possible design of the path command entailed storing a computed shortest path and providing a mechanism to retrieve parts of it later without recomputing the whole thing. However, in general the playfield can be dynamic and obstructions may appear that invalidate a given computed shortest path. Therefore, there can be limited value in caching a computed path for use across ticks.

Instead, in the current implementation a complete path is computed internally upon invoking the path command, and just the direction of the first "move" along that path is returned as a result to the caller.

Max distance

We allow the caller to supply a max distance, but also impose an internal maximum distance to prevent programming errors from irrecoverably freezing the game.