Implement Extend<Bytes> for BytesMut

[robjtede]

Motivation

This lets us play nice with Stream's methods like:

fn get_stream() -> impl Stream<Item = Bytes> { todo!() };
let payload = get_stream().collect::<BytesMut>().await;

fn get_stream() -> impl Stream<Item = Result<Bytes, E>> { todo!() };
let payload = get_stream().try_collect::<BytesMut>().await?;

Since the bound on this collect is a bit different to std's iter collect which is based on FromIterator.

[Darksonn]

Thanks.