Add drop guard for cancellation token

[MikailBag]

Motivation

Usually, cancellation is implemented as dropping a future. However, this does not work when a future internally spawns background tasks.

Solution

A new method drop_guard is added. It returns a special guard which cancels the token unless disarmed.
Intended usage:

async fn my_simple_fut() {
    let token = CancellationToken::new();
    let _g = token.clone().drop_guard();
    tokio::task::spawn(async move {
         some_lib::do_work(..., token);
    });
    // some other work here.
}

[Darksonn]

Besides the doc-comment, this seems fine to me.

[Darksonn]

The documentation could be improved.

[Darksonn]

Any updates on this?

[Darksonn]

Newlines in doc-comments do not translate to newlines in the generated documentation unless there is an empty line.

[MikailBag]

Yeah, i just dislike long lines :)

Do you think splitting first sentence into a separate paragraph makes sense?

[Darksonn]

Sure.

Generally, we wrap documentation at 80 chars to avoid long lines.