[board] Implement a Pieces iterator

This type iterates the pieces in a Position using the BitScanner struct to iterate all the occupied squares of the position's bitboards.

board/src/position.rs

@@ -1,5 +1,8 @@
 // Eryn Wells <eryn@erynwells.me>
 
+mod pieces;
+
+use self::pieces::Pieces;
 use crate::bitboard::BitBoard;
 use crate::piece::{Piece, PiecePlacementError};
 use crate::square::Square;
@@ -81,7 +84,7 @@ impl Position {
         Ok(())
     }
 
-    fn bitboard_for_piece(&self, piece: &Piece) -> &BitBoard {
+    pub(super) fn bitboard_for_piece(&self, piece: &Piece) -> &BitBoard {
         &self.pieces_per_type[piece.color as usize][piece.shape as usize]
     }
 
@@ -89,6 +92,11 @@ impl Position {
         &mut self.pieces_per_type[piece.color as usize][piece.shape as usize]
     }
 
+    fn pieces(&self, color: Color) -> Pieces {
+        Pieces::new(&self, color)
+    }
+}
+
 impl fmt::Debug for Position {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut output = String::new();