Add opposite of stream_reader

[Darksonn]

The tokio::io::stream_reader adapter allows you to convert a Stream<Item=Result<Bytes, Error>> into an AsyncRead. I would like to provide an adapter for the opposite conversion as well.

It is currently already possible to do this using

FramedRead::new(my_async_read, BytesCodec::new()).map_ok(BytesMut::freeze)

from tokio-util, but this is such a common operation that I think we should provide a dedicated adapter in tokio::io as well.

Name suggestion: stream_chunker

[taiki-e]

Done in #2714 / #2788. (seems pr to non-default branch doesn't close issue)