[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/bitboard/bitboard.rs

@@ -53,7 +53,8 @@ impl BitBoard {
     }
 
     pub fn is_set(self, sq: Square) -> bool {
-        !(self & &sq.into()).is_empty()
+        let square_bitboard: BitBoard = sq.into();
+        !(self & square_bitboard).is_empty()
     }
 
     pub fn set_square(&mut self, sq: Square) {
@@ -341,4 +342,10 @@ mod tests {
 
         assert_eq!(a, bitboard![B5, C5, G7, H3]);
     }
+
+    #[test]
+    fn from_square() {
+        assert_eq!(BitBoard::from(Square::A1), BitBoard(0b1));
+        assert_eq!(BitBoard::from(Square::H8), BitBoard(1 << 63));
+    }
 }