[board] Make Board::pieces private

Do not allow direct access to the internal piece set. Update call sites to use
Board API instead.

board/src/board.rs

@@ -15,7 +15,7 @@ pub type FullMoveClock = u32;
 #[derive(Clone, Debug, Default, Eq, PartialEq)]
 pub struct Board {
     pub active_color: Color,
-    pub pieces: PieceSet,
+    pieces: PieceSet,
     pub castling_rights: castle::Rights,
     pub en_passant_target: Option<Square>,
     pub half_move_clock: HalfMoveClock,

board/src/check.rs

@@ -23,7 +23,7 @@ impl Board {
     }
 
     fn king_bitboard(&self, color: Color) -> BitBoard {
-        self.pieces.find_pieces(Piece::king(color))
+        self.find_pieces(Piece::king(color))
     }
 }
 

board/src/fen.rs

@@ -209,7 +209,7 @@ impl FromFenStr for Board {
                 let file = files.next().ok_or(FromFenStrError::MissingPlacement)?;
                 let piece = Piece::from_fen_str(&ch.to_string())?;
 
-                let _ = board.pieces.place(
+                let _ = board.place_piece(
                     piece,
                     Square::from_file_rank(*file, *rank),
                     PlacePieceStrategy::default(),