make OwnRef covariant

while it references the memory the value itself is owned, so that’s the right thing

src/ptr.rs

@@ -5,7 +5,7 @@ use std::{
 	ptr::NonNull,
 };
 
-pub struct OwnRef<'a, T: ?Sized>(Option<NonNull<T>>, PhantomData<&'a mut T>);
+pub struct OwnRef<'a, T: ?Sized>(Option<NonNull<T>>, PhantomData<(&'a mut [u8], T)>);
 
 impl<T: ?Sized> Drop for OwnRef<'_, T> {
 	fn drop(&mut self) {