use (unimplemented) BufWriter in serialized_packed_test
use (unimplemented) BufWriter in serialized_packed_test

maurer/capnproto-rust,dwrensha/capnproto-rust,danburkert/capnproto-rust,placrosse/capnproto-rust,larquin/capnproto-rust,tempbottle/capnproto-rust