[board] Declare three new Display-like traits

- ASCIIDisplay → format a type using ASCII only characters
- UnicodeDisplay → format a type using any Unicode characters
- FENDisplay → format a type for inclusion in a FEN string

board/src/lib.rs

@@ -1,6 +1,7 @@
 // Eryn Wells <eryn@erynwells.me>
 
 mod bitboard;
+mod display;
 mod moves;
 #[macro_use]
 pub mod piece;