support bytemuck

[dovahcrow]

One drawback for this PR is that it lowers the unsafe_code lint from forbid to deny because bytemuck requires unsafe.

[robjtede]

Would #[cfg_attr(not(feature = "bytemuck), forbid(unsafe))] work?

[dovahcrow]

Would #[cfg_attr(not(feature = "bytemuck), forbid(unsafe))] work?

Yeah this should work. But is it too verbose because you also need #[cfg_attr(feature = "bytemuck), deny(unsafe)]? I just feel deny is sufficient since it behaviours same as forbid but allows overridden.

[robjtede]

Signalling forbid is important for consumers who run cargo-geiger and have no need of the bytemuck integration.

[dovahcrow]

@robjtede done. I still leave deny/allow there since unsafe code is still not recommended.