Run clippy on CI

[pitbuster]

First take on #287. I am not sure if running nightly clippy is a requirement for the project (in my experience it is quite unstable), but we can work on that if that's the case.

[waywardmonkeys]

I'm not sure if this should be a new job or just part of the existing test workflow.

[alerque]

Personally (having setup and managed a lot of CI stuff) I would probably not put this in the same workflow as the test and coverage jobs. I would either keep it completely separate like it is or adapt the existing format.yml workflow to include all linting related jobs. There isn't a huge difference between these options. Some tooling (like external badge services) make it a little easier to get the status of a workflow rather than a single job in a milti-job workflow, but it isn't a big deal.