[board] Cave to pressure and implement fmt::Display for Position

It prints a nice diagram!

Now I can make the position module private.

explorer/src/main.rs

@@ -1,5 +1,4 @@
 use board::piece::{Color, Piece, Shape};
-use board::position::DiagramFormatter;
 use board::{Position, Square};
 use clap::{Arg, Command};
 use rustyline::error::ReadlineError;
@@ -80,8 +79,7 @@ fn main() -> Result<(), String> {
     let mut pos = Position::empty();
 
     loop {
-        let diagram = DiagramFormatter::new(&pos);
-        println!("{}", diagram);
+        println!("{}", &pos);
 
         let readline = editor.readline("? ");
         match readline {