Implement redundant mailbox representation

src/board/fen.rs

@@ -25,9 +25,13 @@ pub fn from_fen(fen: &str) -> Result<Game, FenError> {
         halfmove_clock,
         fullmove_counter,
     ));
+
+    let mailbox = Mailbox::new_from_board(&board);
+
     Ok(Game {
         board,
         history: History::new(),
+        mailbox,
     })
 }
 
@@ -127,6 +131,7 @@ fn castling_ability(castling: &str) -> Result<[Castle; 2], FenError> {
 use std::collections::HashMap;
 
 use super::history::History;
+use super::mailbox::Mailbox;
 
 fn en_passant_square(square: &str) -> Result<Option<usize>, FenError> {
     let mut sqr = square.chars();