Add rustfmt.toml configuration and re-format everything + fix basic warnings

[daniel-abramov]

So that contributions from multiple people are more consistent w.r.t. code style and formatting 🙂

Note, that this one does not include all of the cargo clippy fixes since there are quite a few of them that require manual intervention. I did not want to make them part of this PR (since unlike the rest of the changes these would not be automated, hence would require a more rigorous review of whether the user-written code changes anything).

Also, cargo check still produces 3 warnings that I did not fix as they would need some minor changes:

• RemoteTrackPublication::on_subscription_status_changed and RemoteTrackPublication::on_permission_status_changed are pub(crate) and never used within a crate. I was not sure if I'm allowed to remove them (since we don't use them) or if I need to make them pub (that would require making the rest of the things pub as well, for consistency).
• RtcSession::publisher() is pub, but never used (not sure why it complains about it given that it is pub, perhaps it's never exported or something).

Also I added a tiny simple job to check the formatting so that we can keep it consistent.

[theomonnom]

Lgtm, TIL about use_small_heuristics

[theomonnom]

Thanks 🙏
I’ll try to address the other warnings