Remove allowed clippy lints from CI checks and resolve resulting errors

[jessebraham]

In #1134 I "fixed" the clippy checks in CI, however we are still allowing a number of lints which I just did not feel like resolving at that point in time (I had already fixed a lot of them 😅).

We should decide which (if any) lint rules we should allow, and then add a [clippy.lints] section to our cargo manifest to be explicit about these (if required). We must then also update the CI workflow to remove the currently allowed lints, and fix any resulting errors.