Check our usage of `repr(C)` vs. `repr(transparent)`

[madsmtm]

They are slightly different, as noted in the RFC introducing the latter

[madsmtm]

I've verified that we don't use #[repr(C)] on any structs like this.

In general, ideally we'd use #[repr(transparent)] whenever possible, since it plays better with extern types once we get those, but the improper_ctypes lint doesn't work that well with it, so I'll keep #[repr(C)] for now.

[silvanshade]

but the improper_ctypes lint doesn't work that well with it

I wonder if there is an issue upstream about this. Maybe worth creating one if not.

[madsmtm]

Good idea, I've opened rust-lang/rust#115457 for that