Fix clippy errors in tests

[JelteF]

Synopsis

We were only running clippy on our normal code, not tests or examples.

Solution

This starts running clippy on the full codebase by using --all-targets. It
also fixes all issues that this flag found.

[tyranron]

@JelteF I've taken a bit of initiative over your work and did the following adjustments:

1. Reverted removing in tests this line

   #[cfg(not(msrv))] // TODO: Remove once MSRV bumps 1.66 or higher.

   Since we're still on 1.65 MSRV and removal failed the CI.

2. Added brief comments to almost all #[allow(..)] usage in the code. This is something I've started doing recently in the crates I support, because it helps me to reconsider them much quicker when tuning lints for newer Rust versions. When I search for them in the IDE, it looks like this: