[board] Fix some bugs in the starting position

Turns out I was doing the starting position really wrong. In an upcoming commit,
I will implement FEN output for Positions. While doing that work, I found several
issues that were causing the output of the FEN formatter to return garbage.

Implement a handful of unit tests to track down the errors.

Rename Shape::_ascii_representation() → Shape::to_ascii.
Implement to_ascii() on Piece.

board/src/position/position.rs

@@ -33,22 +33,22 @@ impl Position {
 
     /// Return a starting position.
     pub fn starting() -> Self {
-        let white_pieces = [
-            BitBoard::new(0x00FF000000000000),
-            BitBoard::new(0x4200000000000000),
-            BitBoard::new(0x2400000000000000),
-            BitBoard::new(0x8100000000000000),
-            BitBoard::new(0x1000000000000000),
-            BitBoard::new(0x0800000000000000),
+        let black_pieces = [
+            BitBoard::new(0b0000000011111111 << 48),
+            BitBoard::new(0b0100001000000000 << 48),
+            BitBoard::new(0b0010010000000000 << 48),
+            BitBoard::new(0b1000000100000000 << 48),
+            BitBoard::new(0b0000100000000000 << 48),
+            BitBoard::new(0b0001000000000000 << 48),
         ];
 
-        let black_pieces = [
-            BitBoard::new(0xFF00),
-            BitBoard::new(0x0042),
-            BitBoard::new(0x0024),
-            BitBoard::new(0x0081),
-            BitBoard::new(0x0010),
-            BitBoard::new(0x0008),
+        let white_pieces = [
+            BitBoard::new(0b1111111100000000),
+            BitBoard::new(0b0000000001000010),
+            BitBoard::new(0b0000000000100100),
+            BitBoard::new(0b0000000010000001),
+            BitBoard::new(0b0000000000001000),
+            BitBoard::new(0b0000000000010000),
         ];
 
         Self {
@@ -242,7 +242,32 @@ impl fmt::Display for Position {
 
 #[cfg(test)]
 mod tests {
-    use crate::{position, Castle, Color};
+    use crate::{position, Castle, Color, Position, Square};
+
+    #[test]
+    fn piece_on_square() {
+        let pos = test_position![
+            Black Bishop on F7,
+        ];
+
+        let piece = pos.piece_on_square(Square::F7);
+        assert_eq!(piece, Some(piece!(Black Bishop on F7)));
+    }
+
+    #[test]
+    fn piece_in_starting_position() {
+        let pos = Position::starting();
+        println!("{pos}");
+
+        assert_eq!(
+            pos.piece_on_square(Square::H1),
+            Some(piece!(White Rook on H1))
+        );
+        assert_eq!(
+            pos.piece_on_square(Square::A8),
+            Some(piece!(Black Rook on A8))
+        );
+    }
 
     #[test]
     fn king_is_in_check() {