Create CI check for unused_features #115
Description
We allowed the unused_features lint in #114, since it was breaking CI. We still want to take advantage of the lint to detect when we don't use a feature. However, while trying to set it up, I encountered this bug: rust-lang/rust#153523. After it's resolved, we could either re-enable the lint, or build a CI check that runs cargo check --all-features with the lint enabled and thus verifies that we use all features that we enable.