FIX: Use repr(C) MaybeUninit after discussion with RalfJung

We have a recommendation from the unsafe-wg (no rule yet), that
repr(C) for unions should work this way, so that we can cast from the
union type to one of its fields.

src/maybe_uninit.rs

@@ -7,6 +7,7 @@ use std::mem::ManuallyDrop;
 /// this wraps a value that can be wholly or partially uninitialized;
 /// it also has no drop regardless of the type of T.
 #[derive(Copy)]
+#[repr(C)] // for cast from self ptr to value
 pub union MaybeUninit<T> {
     empty: (),
     value: ManuallyDrop<T>,