message: test(server): Use a !Send and !Sync HttpBody in an example
  The default Body type is used in the client in a Pool, so it has to be
  Send. On the server, we can use a !Send type if the executor is single
  threaded.
  Expand the existing example to show that.
diff: use std::cell::Cell;
  use std::rc::Rc;
  +use hyper::body::{Bytes, HttpBody};
  +use hyper::header::{HeaderMap, HeaderValue};
  use hyper::service::{make_service_fn, service_fn};
  -use hyper::{Body, Error, Response, Server};
  +use hyper::{Error, Response, Server};
  +use std::marker::PhantomData;
  +use std::pin::Pin;
  +use std::task::{Context, Poll};
  +
  +struct Body {
  + // Our Body type is !Send and !Sync:
  + _marker: PhantomData<*const ()>,
  + data: Option<Bytes>,
  +}
  +
  +impl From<String> for Body {
  + fn from(a: String) -> Self {
  + Body {
  + _marker: PhantomData,
  + data: Some(a.into()),
  + }
  + }
  +}
  +
  +impl HttpBody for Body {
  + type Data = Bytes;
  + type Error = Error;
  +
  + fn poll_data(
  + self: Pin<&mut Self>,
  + _: &mut Context<'_>,
  + ) -> Poll<Option<Result<Self::Data, Self::Error>>> {
  + Poll::Ready(self.get_mut().data.take().map(Ok))
  + }
  +
  + fn poll_trailers(
  + self: Pin<&mut Self>,
  + _: &mut Context<'_>,
  + ) -> Poll<Result<Option<HeaderMap<HeaderValue>>, Self::Error>> {
  + Poll::Ready(Ok(None))
  + }
  +}
  fn main() {
  pretty_env_logger::init();
  