Support `MaybeUninit` and `ManuallyDrop`

[42triangles]

I think these should really be supported, as it is can be somewhat important when dealing with a C API. Since both are #[repr(transparent)], and since (afaik) C fields do not have to be initialized as long as they aren't being used, MaybeUninit<T> and ManuallyDrop<T> should probably be exopsed as whatever T would translated to on its own.

[emilio]

This makes sense. We already hande other standard types, so I don't think it should be too terrible to fix.

[chrysn]

Would that allow writing C functions as pub extern "C" fn my_fun(x: &mut MaybeUninit<somestruct>)?

I'm trying to follow along the lines of what's there for NonNull and Cell, maybe I can come up with a PR.

[emilio]

Yes it would.

[emilio]

And yeah, doing what Cell does sounds alright.

[cceckman]

If I'm this comment, this is closed as of 0.15.0 (af6d2e3 in tree). @emilio / @Hywan can you confirm?

(Thanks for adding this!)

[emilio]

Yes, that's right!