Update the book tutorial to use async_std channels

[yoshuawuyts]

With #380 landed, we should update the tutorial to use them. This would remove the final dependency on futures-rs (and importantly: Sink) which should be a big step up in terms of usability.

Also we should consider moving from select! {} to Stream::merge. From chat with @skade we've figured out how to "send a close message after a stream has been exhausted":

enum EventKind {
    Event(/* event_type */),
    Shutdown,
}

let events = events.map(|ev| EventKind::Event(ev)).join(stream::once(EventKind::Shutdown));
let shutdown = shutdown.map(|e| EventKind::Event(e));
let s = events.merge(shutdown);

while let Some(event) = s.next().await {
    match event {
        EventKind::Event(ev) => /* handle event */,
        EventKind::Shutdown => /* handle shutdown */,
    }
}

@skade I'm assigning this to you as you've been working on updating this recently. Feel free to unassign yourself if you don't think you'll have bandwidth. Thanks!

edit: this would resolve #105 and #13

[matklad]

cc me: I am slightly worried about introduction of explicit Shutdown messages, as opposed to just exhausting the stream. I wan to say that I want to port the tutorial myself, but I am in the middle of big refactoring of rust-analyzer right now. I'd be interested in reviewing the update PR!

[skade]

See #419 for a draft

[matklad]

Figured out a way to get rid of match event { Shutdown => } smell by introducing a hof which nips a stream based on a side-channel: #419 (comment)