test: Test crates separately with relevant combinations of feature flags

The CI currently uses cargo test --workspace, but I've noticed that this does not catch bugs affecting tests run on an individual crate.

Improve my testing workflow & CI to catch bugs that may only appear when testing crates individually.

Also test different feature flag settings or combinations.

Possibly useful (at cursory glance, to explore):

• https://github.com/taiki-e/cargo-hack