general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

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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Move all to top level

remove transport and merge it to util

update util with stable tokio 0.2

finished Stream/SessionSRTP part

WIP: ???

fix async version of buffer test

add async buffer tests

pass the first async test