util: add writev-aware poll_write_buf (#3156)

## Motivation

In Tokio 0.2, `AsyncRead` and `AsyncWrite` had `poll_write_buf` and
`poll_read_buf` methods for reading and writing to implementers of
`bytes` `Buf` and `BufMut` traits. In 0.3, these were removed, but
`poll_read_buf` was added as a free function in `tokio-util`. However,
there is currently no `poll_write_buf`.

Now that `AsyncWrite` has regained support for vectored writes in #3149,
there's a lot of potential benefit in having a `poll_write_buf` that
uses vectored writes when supported and non-vectored writes when not
supported, so that users don't have to reimplement this.

## Solution

This PR adds a `poll_write_buf` function to `tokio_util::io`, analogous
to the existing `poll_read_buf` function.

This function writes from a `Buf` to an `AsyncWrite`, advancing the
`Buf`'s internal cursor. In addition, when the `AsyncWrite` supports
vectored writes (i.e. its `is_write_vectored` method returns `true`),
it will use vectored IO.

I copied the documentation for this functions from the docs from Tokio
0.2's `AsyncWrite::poll_write_buf` , with some minor modifications as
appropriate.

Finally, I fixed a minor issue in the existing docs for `poll_read_buf`
and `read_buf`, and updated `tokio_util::codec` to use `poll_write_buf`.

Signed-off-by: Eliza Weisman <[email protected]>

tokio-util/src/io/read_buf.rs

@@ -5,9 +5,9 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 use tokio::io::AsyncRead;
 
-/// Read data from an `AsyncRead` into an implementer of the [`Buf`] trait.
+/// Read data from an `AsyncRead` into an implementer of the [`BufMut`] trait.
 ///
-/// [`Buf`]: bytes::Buf
+/// [`BufMut`]: bytes::BufMut
 ///
 /// # Example
 ///