FIX: Use repr(C) MaybeUninitCopy 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_copy.rs

@@ -2,6 +2,7 @@
 use array::Array;
 
 #[derive(Copy, Clone)]
+#[repr(C)] // for cast from self ptr to value
 pub union MaybeUninitCopy<T>
     where T: Copy
 {