Add sending trailers on Body channel

[seanmonstar]

The Body::channel() variant is a common simple way to send data as a Body. It would be useful to also be able to send trailers on it.

Implementation

The code to change is on the hyper::body::Sender, and the relevant Kind variant. I could imagine two possible ways to implement this, and I'm not sure which is better yet.

1. Change the internal mpsc channel to send enum Msg { Data(Bytes), Trailers(HeaderMap) }. However, the Kind::Chan variant would need to grow some cache slots, since poll_data could pop a Msg::Trailers, and then that would need to cached so that poll_trailers returns its.
2. Add a oneshot channel to send the trailers. The downside here is the increased size of Sender and Kind::Channel (they both are carrying an extra pointer), and the extra allocation.

I think the 2nd option is probably going to be better, but measurements are king. We should compare the changes to the benches/end_to_end results to see if the added feature makes much of a difference. If it does, we could consider making an internal "skinnier channel" that is used by the h1 dispatcher.