[explorer, position] Make Position.board private to the crate

Export a Position::board() method that returns a reference to the internal Board.

position/tests/peterellisjones.rs

@@ -8,7 +8,7 @@
 
 use chessfriend_core::Color;
 use chessfriend_moves::{
-    assert_move_list, assert_move_list_contains, assert_move_list_does_not_contain, ply, Move,
+    Move, assert_move_list, assert_move_list_contains, assert_move_list_does_not_contain, ply,
 };
 use chessfriend_position::test_position;
 use std::collections::HashSet;
@@ -107,7 +107,7 @@ fn en_passant_check_capture() {
         White Pawn on D4,
     ], D3);
 
-    assert!(pos.board.active_color_is_in_check());
+    assert!(pos.board().active_color_is_in_check());
 
     let generated_moves: HashSet<_> = pos.all_legal_moves(Some(Color::Black)).collect();
 
@@ -123,7 +123,7 @@ fn en_passant_check_block() {
         White Queen on F1,
     ], D3);
 
-    assert!(pos.board.active_color_is_in_check());
+    assert!(pos.board().active_color_is_in_check());
 
     let generated_moves: HashSet<_> = pos.all_legal_moves(Some(Color::Black)).collect();
 
@@ -139,7 +139,7 @@ fn pinned_pieces_rook_cannot_move_out_of_pin() {
         White King on C1,
     ]);
 
-    assert!(!pos.board.active_color_is_in_check());
+    assert!(!pos.board().active_color_is_in_check());
 
     let rook_moves: HashSet<_> = pos.all_legal_moves(None).collect();