Fix or allow all Clippy errors, re-enable dead code and unused variable warnings

[Rua]

Changelog:

- Improvements to compiler linting:
  - Most clippy warnings and errors have been fixed, and the remainder is explicitly allowed in each crate root. Some of these may be re-enabled in the future if desired.
  - Warnings for dead code and unused variables have been re-enabled and fixed.

A huge number of changed files here, but nothing has been changed semantically (hopefully). This makes clippy actually usable, and also re-enables the warnings for dead code and unused variables, which were disabled but with a note to re-enable them at some point. I explicitly disabled the clippy warnings that I felt were too pedantic or opinionated.

Fixes #1195.

[AustinJ235]

Looks good. Didn't notice any semantic changes. Great work!