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_range_loop` warning in tests

Simplify boolean expressions

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!()`

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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Fix clippy for rust 1.66 (#369)

PollStream/PollDataChannel: flush before shutting down (#340)

* PollStream/PollDataChannel: flush before shutting down

"Invocation of a shutdown implies an invocation of flush. Once this
method

PollStream/PollDataChannel: flush before shutting down (#340)

* PollStream/PollDataChannel: flush before shutting down

"Invocation of a shutdown implies an invocation of flush. Once this
method returns Ready it implies that a flush successfully happened
before the shutdown happened. That is, callers don’t need to call flush
before calling shutdown. They can rely that by calling shutdown any
pending buffered data will be written out."

https://docs.rs/tokio/1.21.2/tokio/io/trait.AsyncWrite.html#tymethod.poll_shutdown

* format code

* fix clippy warning

* fix another clippy warning

* add changelog entries

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Clippy fix

Move all to top level