138: Fix some clippy lints r=matklad a=alanhdu
I went ahead and fixed all the clippy lints (there were a couple I thought would be better unfixed and added `cfg` statements to allow them) and also re-enabled clippy and rustfmt in CI.
They were disabled with `no time to explain, disable clippy checks`, so hopefully this won't go against whatever the reason at the time was 😆.
One question about the CI though: right now, it's an allowed failure that runs against the latest nightly each time. Would it be better to pin it to a specific nightly (or use the `beta` versions) to lower the churn?
Co-authored-by: Alan Du <alanhdu@gmail.com>
