[board] Implement a DiagramFormatter that writes a graphical diagram of a Position

Remove fmt::Display from Position and move display to a DiagramFormatter type that implements that trait and writes a position to a formatter.

board/src/position/mod.rs

@@ -1,5 +1,7 @@
+mod diagram_formatter;
 mod pieces;
 mod position;
 
+pub use diagram_formatter::DiagramFormatter;
 pub use pieces::Pieces;
 pub use position::Position;