Configure CI and fix clippy warnings

[Shatur]

I set up dependabot (to automatically update dependencies), cargo deny (to check dependencies for vulnerabilities and license compatibility), formatting, tests and clippy checks.
Also I fixed discovered warnings (all changes was done only to silence warnings), applied formatting (I left only deprecated std::env::home_dir() function, probably should be replaced with dirs-rs) to make CI pass.

Note: I had to replace new with default and new_with_names with just new due to this warning. And I think this actually makes sense :)

[mentaljam]

Thanks for your work! All the changes look valuable for me. And I'm glad that this crate is still in use🙂