Address clippy warnings

[kailan]

For as long as I can remember, cargo clippy has produced a huge amount of warnings in this project. This pull request resolves as many of those warnings as possible, with a few exceptions:

• Large size difference between enum variants
• Functions with too many arguments
• Complex type definition

I felt these were out of scope for a large sweeping change like this so I've annotated them to be ignored by clippy.

As a result, clippy output is now clean, and it can be added to the CI workflow to ensure new changes are checked for best practices.

[kpfleming]

Overall there are a ton of improvements here, including use of if-let instead of single-arm match, use of let chains instead of nested if blocks, and similar things. I've left a couple of comments on changes that I'm not sure I fully understand and which will need review from others.

There are probably more improvements like this to be had if we allow Viceroy to be built with a newer version of Rust, it's still pinned to 1.90.

[kailan]

There are probably more improvements like this to be had if we allow Viceroy to be built with a newer version of Rust, it's still pinned to 1.90.

I switched to Rust 1.95 and ran clippy there, which found a few more improvements that I included in this PR. For making that upgrade, I have #604 open which is based on this branch.

[cceckman-at-fastly]

Generally looks good.

I'd rather get the ignored lints fixed than allowed; #607 addresses those.