Add standard clippy lints and run them on all Rust code in CI

[teor2345]

This PR:

• adds some standard clippy lints (based on Zebra)
• runs these lints on all code in CI

The full list of clippy lints and descriptions is here: https://rust-lang.github.io/rust-clippy/stable/

This is compatible with PR #4, it doesn't matter which order they merge in. (And there shouldn't be any merge conflicts.)