[board] Implement PositionBuilder::from_position

Create a PositionBuilder from a Position.

board/src/position/builders/position_builder.rs

@@ -25,6 +25,27 @@ impl Builder {
         Self::default()
     }
 
+    pub fn from_position(position: &Position) -> Self {
+        let pieces = BTreeMap::from_iter(
+            position
+                .pieces(Color::White)
+                .chain(position.pieces(Color::Black))
+                .map(|placed_piece| (placed_piece.square(), *placed_piece.piece())),
+        );
+
+        let white_king = position.king_square(Color::White);
+        let black_king = position.king_square(Color::Black);
+
+        Self {
+            player_to_move: position.player_to_move(),
+            flags: position.flags(),
+            pieces,
+            kings: [white_king, black_king],
+            ply_counter: position.ply_counter(),
+            move_number: position.move_number(),
+        }
+    }
+
     pub fn to_move(&mut self, player: Color) -> &mut Self {
         self.player_to_move = player;
         self