[board, explorer, moves] Make Board::active_color private

Make the struct attribute private, and export two new methods. A getter, active_color(),
and a setter, set_active_color().

Update all references to the attribute to use the methods.

board/src/macros.rs

@@ -13,7 +13,7 @@ macro_rules! test_board {
                 chessfriend_core::Square::$square,
                 $crate::PlacePieceStrategy::default());
             )*
-            board.active_color = chessfriend_core::Color::$to_move;
+            board.set_active_color(chessfriend_core::Color::$to_move);
             board.en_passant_target = Some(chessfriend_core::Square::$en_passant);
 
             println!("{}", board.display());