feat: apply clippy::pedantic lints & improve CI

[bryango]

This PR provides:

• more thorough testing via github actions, and
• some functionally trivial changes to improve the rust codebase, guided by the clippy::pedantic lints.

It would be easy to review commit by commit:

1. the first commit sets up the clippy lints and github actions CI, while
2. the second commit actually applies the clippy lints by cargo clippy --fix with some manual tweaks for readability.
3. (new) the third commit drops magic nix cache in CI as it's near its end of life: https://determinate.systems/posts/magic-nix-cache-free-tier-eol/

After this, I think we have collected enough minor fixes to justify a new bugfix release, as is hoped for in #60 (comment) 😆

[bryango]

Update: added a third commit which drops magic nix cache in CI as it's near its end of life: https://determinate.systems/posts/magic-nix-cache-free-tier-eol/

[bryango]

These are mostly trivial changes so I will be merging this soon, if there are no objections! 👀 @makefu @Artturin I will also make a bugfix release afterward for #60.