Implement std::fmt::Display for board

rs/src/main.rs

@@ -11,7 +11,7 @@ fn main() -> Result<(), ()> {
         match input_lines.next() {
             None => break Ok(()),
             Some(line) => match &line.unwrap()[..] {
-                "get_state" => println!("{}", board.to_string()),
+                "get_state" => println!("{}", board),
                 _ => break Err(()),
             },
         }