DuplexStream does not support vectored writes

[qrnch-jan]

When write_vectored is used on a DuplexStream it only writes the first buffer in the scatter/gather list.

It would be nice if it wrote as much as it can.

The following code illustrates the problem:

use std::io::IoSlice;
use bytes::Bytes;
use tokio::io::AsyncWriteExt;

#[tokio::main]
async fn main() {
    let bufs = vec![Bytes::from("hello,"), Bytes::from(" world")];
    let (mut ep1, _ep2) = tokio::io::duplex(64*1024);
    let iov: Vec<IoSlice> = bufs.iter().map(|b| IoSlice::new(b)).collect();
    println!("iov.len={}", iov.len());
    let total_size = iov.iter().fold(0, |acc, e| acc + e.len());
    println!("total size: {}", total_size);
    let written = ep1.write_vectored(&iov[..]).await.unwrap();
    println!("wrote {} bytes", written)
}

The program outputs that it has written 6 bytes, rather than the expected 12.