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

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

explorer/src/main.rs

@@ -187,7 +187,7 @@ fn respond(line: &str, state: &mut State) -> anyhow::Result<CommandResult> {
 }
 
 fn do_flags_command(state: &mut State, _matches: &clap::ArgMatches) -> CommandResult {
-    let board = &state.position.board;
+    let board = state.position.board();
 
     println!("Castling:");
 
@@ -390,7 +390,7 @@ 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.active_color());
         }
 
         let readline = editor.readline("\n? ");