general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

Silence `clippy::needless_update` warning in tests

Replace `assert_eq!(<bool>, true)` with `assert!(<bool>)` and `assert_eq!(<bool>, false)` with `assert!(!<bool>)`

Replace `assert!(false, …)`/`assert!(false)` with `panic!(…)`/`panic!()`

Move all to top level