Rollup merge of #161328 - ChrisDenton:clippy3, r=nia-e

Enforce even more library clippy lints in CI

Once again, this is best reviewed commit-by-commit since each commit fixes a lint.

I've bundled together a large(ish) number of lints because I feel they shouldn't be controversial and don't touch too much code. A large chunk of them is just telling clippy that, for example, implementing `is_digit` by calling `is_digit` just isn't going to work. I have reviewed each and every fix myself but please do double or triple check my working!