[board, position] Simplify check methods

Only one check-testing method, Board::is_in_check(), that tests if the current
active player is in check. It doesn't make sense to test if the non-active player
is in check.

position/src/position.rs

@@ -129,7 +129,7 @@ impl Position {
                     );
                 });
 
-            let move_is_legal = !test_board.color_is_in_check(Some(active_color_before_move));
+            let move_is_legal = !test_board.is_in_check();
 
             test_board.unmake_move(&record).unwrap_or_else(|err| {
                 panic!(

position/tests/peterellisjones.rs

@@ -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().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().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().is_in_check());
 
     let rook_moves: HashSet<_> = pos.all_legal_moves(None).collect();