Consider running cargo-hack and cargo-unused-features in CI

[smalis-msft]

We don't really have a great story today for ensuring that our feature dependencies are 100% correct, both in terms of building crates in isolation or non-typical scenarios, and in terms of being truly minimal. There are tools out there that claim to be able to detect these cases, namely https://crates.io/crates/cargo-hack and https://crates.io/crates/cargo-unused-features (and maybe others?). I don't think running these on PR gates makes sense given their likely long runtimes over our large codebase, but maybe running them on a daily job would make sense?

[smalis-msft]

There's also apparently the unused-crate-dependencies lint, which can get much more precise than our current cargo-machete-ish approach, but requires actually building. Combining it with cargo-hack could be nice, but would probably take some work to get clean.