message: Make CopyStream !Unpin
diff: use crate::client::{InnerClient, Responses};
  +use pin_project_lite::pin_project;
  use crate::codec::FrontendMessage;
  use crate::connection::RequestMessages;
  use crate::types::ToSql;
  @@ -8,6 +9,7 @@ use futures::{ready, Stream};
  use postgres_protocol::message::backend::Message;
  use std::pin::Pin;
  use std::task::{Context, Poll};
  +use std::marker::PhantomPinned;
  pub async fn copy_out<'a, I>(
  client: &InnerClient,
  @@ -20,7 +22,7 @@ where
  {
  let buf = query::encode(client, &statement, params)?;
  let responses = start(client, buf).await?;
  - Ok(CopyStream { responses })
  + Ok(CopyStream { responses, _p: PhantomPinned })
  }
  async fn start(client: &InnerClient, buf: Bytes) -> Result<Responses, Error> {
  @@ -39,16 +41,22 @@ async fn start(client: &InnerClient, buf: Bytes) -> Result<Responses, Error> {
  Ok(responses)
  }
  +pin_project! {
  /// A stream of `COPY ... TO STDOUT` query data.
  pub struct CopyStream {
  responses: Responses,
  + #[pin]
  + _p: PhantomPinned,
  + }
  }
  impl Stream for CopyStream {
  type Item = Result<Bytes, Error>;
  - fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
  - match ready!(self.responses.poll_next(cx)?) {
  + fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
  + let this = self.project();
  +
  + match ready!(this.responses.poll_next(cx)?) {
  Message::CopyData(body) => Poll::Ready(Some(Ok(body.into_bytes()))),
  Message::CopyDone => Poll::Ready(None),
  _ => Poll::Ready(Some(Err(Error::unexpected_message()))),
  