general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

Silence `clippy::type_complexity` warning in tests

Replace if-let with `Ok(())` & `Err(err)` arms on result with `result.map(|_| ())`

Replace `Box::new(T::default())` with `Box::<T>::default()`

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

Unify `assert_eq!(…)`/`assert_ne!(…)` to take args in order of `…!(actual, expected)`

While there isn't an explicit convention for this in Rust the stdlib docs all use `assert_eq!(actual, expected)`

Unify `assert_eq!(…)`/`assert_ne!(…)` to take args in order of `…!(actual, expected)`

While there isn't an explicit convention for this in Rust the stdlib docs all use `assert_eq!(actual, expected)`, making it a sort of implicit convention.

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