[board, position] Return replaced piece when placing a piece with PlacePieceStrategy::Replace

When place_piece() is called with PlacePieceStrategy::Replace, return the replaced
piece in the Ok() variant of the Result as an Option<Piece>.

Plumb this new return type through Board and Position.

position/src/position.rs

@@ -59,7 +59,7 @@ impl Position {
         piece: Piece,
         square: Square,
         strategy: PlacePieceStrategy,
-    ) -> Result<(), PlacePieceError> {
+    ) -> Result<Option<Piece>, PlacePieceError> {
         self.board.place_piece(piece, square, strategy)
     }