feat(lib): update bytes to 0.6, update http-body (#2339)

This branch updates `bytes` and `http-body` to the latest versions. The
`http-body` version that uses `bytes` 0.6 hasn't been released yet, so
we depend on it via a git dep for now. Presumably Hyper and `http-body`
will synchronize their releases.

Other than that, this is a pretty mechanical update. Should fix the
build and unblock the `h2` update to use vectored writes.

src/proto/h1/io.rs

@@ -3,6 +3,7 @@ use std::cmp;
 use std::fmt;
 use std::io::{self, IoSlice};
 use std::marker::Unpin;
+use std::mem::MaybeUninit;
 
 use bytes::{Buf, BufMut, Bytes, BytesMut};
 use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};
@@ -191,7 +192,10 @@ where
         if self.read_buf_remaining_mut() < next {
             self.read_buf.reserve(next);
         }
-        let mut buf = ReadBuf::uninit(&mut self.read_buf.bytes_mut()[..]);
+
+        let dst = self.read_buf.bytes_mut();
+        let dst = unsafe { &mut *(dst as *mut _ as *mut [MaybeUninit<u8>]) };
+        let mut buf = ReadBuf::uninit(dst);
         match Pin::new(&mut self.io).poll_read(cx, &mut buf) {
             Poll::Ready(Ok(_)) => {
                 let n = buf.filled().len();