Use Pin<Box<S>> in BodyStream and SizedStream (#1328)

Fixes #1321

A better fix would be to change `MessageBody` to take a `Pin<&mut
Self>`, rather than a `Pin<&mut Self>`. This will avoid requiring the
use of `Box` for all consumers by allowing the caller to determine how
to pin the `MessageBody` implementation (e.g. via stack pinning).

However, doing so is a breaking change that will affect every user of
`MessageBody`. By pinning the inner stream ourselves, we can fix the
undefined behavior without breaking the API.

I've included @sebzim4500's reproduction case as a new test case.
However, due to the nature of undefined behavior, this could pass (and
not segfault) even if underlying issue were to regress.

Unfortunately, until rust-lang/unsafe-code-guidelines#148 is resolved,
it's not even possible to write a Miri test that will pass when the bug
is fixed.

Co-authored-by: Yuki Okushi <huyuumi.dev@gmail.com>

tests/test_weird_poll.rs

@@ -0,0 +1,26 @@
+// Regression test for #/1321
+
+use futures::task::{noop_waker, Context};
+use futures::stream::once;
+use actix_http::body::{MessageBody, BodyStream};
+use bytes::Bytes;
+
+#[test]
+fn weird_poll() {
+    let (sender, receiver) = futures::channel::oneshot::channel();
+    let mut body_stream = Ok(BodyStream::new(once(async {
+        let x = Box::new(0);
+        let y = &x;
+        receiver.await.unwrap();
+        let _z = **y;
+        Ok::<_, ()>(Bytes::new())
+    })));
+
+    let waker = noop_waker();
+    let mut context = Context::from_waker(&waker);
+
+    let _ = body_stream.as_mut().unwrap().poll_next(&mut context);
+    sender.send(()).unwrap();
+    let _ = std::mem::replace(&mut body_stream, Err([0; 32])).unwrap().poll_next(&mut context);
+}
+