Add some more miri tests

I'm not sure this scattering of #[cfg(not(miri))] and #![forbid(unsafe_code)] is the best approach. But I can't think of a better one right now.

Ideally we would have something which would

• check that every #[cfg(not(miri))] is within a wider #![forbid(unsafe_code)]
• check that every crate that has any unsafe is tested with miri

I'm very unsure how to do either of those, let alone both. This pattern will do for now, I guess.