clippy: require must_use_candidate lint

[phip1611]

Adds the clippy::must_use_candiate-lint. Luckily, this was easy as cargo clippy --fix worked like a charm.

I had to do some minor adjustments to the device path code generation functionality.

Closes #565.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)