chore: fix clippy lints and cargo check warnings

[sd2k]

Mostly a 'cargo clippy --fix --all-features --all-targets',
plus some manual fixes for elided lifetimes.

[sd2k]

Looks like a transient dep has a higher MSRV 😞

[sunfishcode]

Thanks for the PR!

The files tests/fs_utf8.rs, tests/net-udp.rs, and others that start with a comment "This file is derived from ..." are derived from other sources. In those files, it's more desirable to minimize diffs with the upstream sources than to fix clippy lints, so please revert the clippy changes in those files. Adding #![clippy::allow(...)] directives at the top of those files would be ok though.

[sd2k]

I think most of these lints have actually been fixed on main now, and the remaining ones are almost all in derived files, so this can be closed 🙂