Address compiler warnings given by `cargo check` or `cargo build`.

Address coding style issues given by `cargo fmt`.

Address issues given by linter, `cargo clippy`.

* remove unnecessary return.
* remove unnecessary to_string.
* remove comparison of bool with true/false
* remove unnecessary reference
* remove redundant use tokio
* remove unnecessary let binding
* remove empty strings in println!()
* use vec![] macro instead of pushing values to a mutable Vec
* use &str instead of &String to avoid creating a new object

Add new GitHub Actions workflow `ci.yaml`, which runs `cargo fmt`,
`cargo build`, `cargo test`, and `cargo clippy`.