rust private

Additional traits for std::mem::ManuallyDrop The first commit adds `Clone` and `Copy` trait implementations for `ManuallyDrop`. Although `Drop` and `Copy` cannot be used together, this may be useful for generics. The second commit adds implementations common traits. I do not think this is necessary, as they could be implemented in a wrapper type outside the standard library, but it would make `ManuallyDrop` more convenient to use.