[board] Implement a rather dumb Position::piece_on_square

This method does an iteration over all piece colors and shapes to find the piece on a given square.

board/src/position/position.rs

@@ -2,7 +2,7 @@
 
 use super::Pieces;
 use crate::bitboard::BitBoard;
-use crate::piece::{Color, Piece, PiecePlacementError};
+use crate::piece::{Color, Piece, PiecePlacementError, PlacedPiece, Shape};
 use crate::Square;
 use std::fmt;
 use std::fmt::Write;
@@ -98,6 +98,20 @@ impl Position {
         &mut self.pieces_per_type[piece.color as usize][piece.shape as usize]
     }
 
+    pub fn piece_on_square(&self, sq: Square) -> Option<PlacedPiece> {
+        for color in Color::iter() {
+            for shape in Shape::iter() {
+                let piece = Piece::new(color, shape);
+                let bb = self.bitboard_for_piece(&piece);
+                if bb.has_piece_at(&sq) {
+                    return Some(PlacedPiece::new(piece, sq));
+                }
+            }
+        }
+
+        None
+    }
+
     pub fn pieces(&self, color: Color) -> Pieces {
         Pieces::new(&self, color)
     }