Check `cfg` to catch feature typos

[BD103]

Nightly Rust has added support for -Zcheck-cfg, which will show a warning if there is a #[cfg(...) for a nonexistant feature / cfg.

While not possible to add this to CI because it is currently nightly-only, it can be run locally to catch all current typos.

Please read this comment on how to use -Zcheck-cfg. Once you follow the setup, run:

cargo +nightly check -Zcheck-cfg --workspace

I found a few warnings related to missing features, but I currently don't have the time to work on it today.

[alice-i-cecile]

We can actually add this to CI: we run some checks on nightly already :)

[eerii]

I'm working on removing the warnings and adding the check to the CI. I'll make a pr in a bit once it is ready!