[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.

explorer/src/main.rs

@@ -283,7 +283,7 @@ fn main() -> Result<(), String> {
     loop {
         if should_print_position {
             println!("{}", &state.position);
-            println!("{} to move.", state.position.board.active_color);
+            println!("{} to move.", state.position.board.active_color());
         }
 
         let readline = editor.readline("\n? ");