Apply clippy fixes, except in #[test] code

[bossmc]

This also adds a CI stage to run clippy over the project too, hopefully this will prevent degradation.

[bossmc]

Build failure was due to my rustc allowing nightly features by default, I've marked the benches that use #![feature(test)] as also needing #![cfg(feature = nightly)] so they won't get built without the nightly feature enabled.

I've also rebased to pick up the recent master changes.

[FrankReh]

I will add it's nice if the project would pass clippy cleanly because it makes playing around with the package easier for a new-comer.