general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

Simplify initialization of types with `Default::default()`, followed by field assignment in tests

Remove needless use of mutable references where an immutable one suffices
for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_mut_passed

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

Add method to delete `allocation` by `username` (#263)

Move all to top level