This commit adds `poll_write_buf` and `write_buf` functions to
`tokio_util::io`, analogous to the existing `poll_read_buf` and
`read_buf` functions.

These functions write 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`),
they will use vectored IO.

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

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Instead, we'll add vectored write support to `tokio`'s
`AsyncWriteExt::write_buf` method. This PR only adds `poll_write_buf`
in `tokio-util`.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>