async-await: use new PinMut/PinBox location (#613)

The types moved in `std`. This patch updates tokio-async-await to
import `PinMut` and `PinBox` from the new location.

Ref: rust-lang/rust#53227

tokio-async-await/src/async_await/compat/backward.rs

@@ -4,7 +4,7 @@ use futures::{
 };
 use futures_core::{Future as Future03};
 
-use std::boxed::PinBox;
+use std::pin::PinBox;
 use std::future::FutureObj;
 use std::ptr::NonNull;
 use std::task::{

tokio-async-await/src/async_await/compat/forward.rs

@@ -4,7 +4,7 @@ use futures_core::future::Future as Future03;
 use futures_core::task::Poll as Poll03;
 
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 use std::task::Context;
 
 /// Converts an 0.1 `Future` into an 0.3 `Future`.

tokio-async-await/src/async_await/io/flush.rs

@@ -5,7 +5,7 @@ use futures_core::task::{self, Poll};
 
 use std::io;
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 
 /// A future used to fully flush an I/O object.
 #[derive(Debug)]

tokio-async-await/src/async_await/io/read.rs

@@ -5,7 +5,7 @@ use futures_core::task::{self, Poll};
 
 use std::io;
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 
 /// A future which can be used to read bytes.
 #[derive(Debug)]

tokio-async-await/src/async_await/io/read_exact.rs

@@ -6,7 +6,9 @@ use futures_util::try_ready;
 
 use std::io;
 use std::marker::Unpin;
-use std::mem::{self, PinMut};
+use std::mem;
+
+use core::pin::PinMut;
 
 /// A future which can be used to read exactly enough bytes to fill a buffer.
 #[derive(Debug)]

tokio-async-await/src/async_await/io/write.rs

@@ -5,7 +5,7 @@ use futures_core::task::{self, Poll};
 
 use std::io;
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 
 /// A future used to write data.
 #[derive(Debug)]

tokio-async-await/src/async_await/io/write_all.rs

@@ -6,7 +6,9 @@ use futures_util::try_ready;
 
 use std::io;
 use std::marker::Unpin;
-use std::mem::{self, PinMut};
+use std::mem;
+
+use core::pin::PinMut;
 
 /// A future used to write the entire contents of a buffer.
 #[derive(Debug)]

tokio-async-await/src/async_await/sink/send.rs

@@ -4,7 +4,7 @@ use futures_core::future::Future;
 use futures_core::task::{self, Poll};
 
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 
 /// Future for the `SinkExt::send_async` combinator, which sends a value to a
 /// sink and then waits until the sink has fully flushed.

tokio-async-await/src/async_await/stream/next.rs

@@ -3,7 +3,7 @@ use futures_core::future::Future;
 use futures_core::task::{self, Poll};
 
 use std::marker::Unpin;
-use std::mem::PinMut;
+use std::pin::PinMut;
 
 /// A future of the next element of a stream.
 #[derive(Debug)]