Add interval api.

[dvc94ch]

Based on #35, needed to use async-io in more places in libp2p.

[dvc94ch]

Looks like ci is having issues (https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/)

[dvc94ch]

@stjepang any comments? It's currently blocking a PR of mine.

[ghost]

Thanks for implementing this! The PR looks good, I would just request the following changes:

1. Implement poll() by calling poll_next(). Treat a regular timer (at()/after()) as a stream (interval_at()) with a period of infinity.
2. Add a unit test to tests/timer.rs

[ghost]

Why is the first tick of a timer constructed by interval() immediate? setInterval() in javscript and Ticker in Go don't fire immediately, so I'm inclined to follow the same behavior here...

[dvc94ch]

I think I agree, you can easily get the first tick with Timer::interval_at(Instant::now(), period)

[ghost]

Thank you!

[dvc94ch]

Can you release 1.3.0? Then I can update my PR's to libp2p.