Replace async-mutex with async-lock

[jayvdb]

Also apply cargo clippy --fix

[museun]

Thanks, but I should mention that this crate is basically unmaintained.

I started work on a sans-io approach (https://github.com/museun/twitch_message) but I haven't had the time to maintain either.

I could merge this, it seems rather simple. But I don't know when I'll publish a new release that includes it.

[jayvdb]

It doesnt bother me if this isnt released any time soon.