Investigate if running aggresive `clippy --all-features` is feasible

[berkus]

TL;DR:

1. We run clippy --all-features on stable, risking build failures if new features eventually require nightly.
2. We run clippy --all-features on nightly, but that might require lint workarounds.
3. We keep it as is and only require clippy once a feature has been stabilized.

I went with option 3, but currently the pipeline will pass for all three options. WDYT?

Originally posted by @berestovskyy in #57 (comment)