Add empty_position()

src/main.rs

@@ -24,6 +24,10 @@ impl World {
             Orientation::W => '←',
         }
     }
+    /// Check if a position is free.
+    fn empty_position(&mut self, p: Position) -> bool {
+        self.map[(p.x * p.y) as usize] == '.'
+    }
 }
 
 /// Struct to store robot position.