Add `Show` and `Clone` trait to arrays

Due to not being able to parametrize over array sizes, `Clone` is only
implemented for element types that are `Copy`able.
That's not safe because `clone` can throw, and calling a destructor on zeroed memory is not well-defined. Drop flags are an implementation detail and removing them is planned. It *is* possible to write this code correctly, but you need to handle the unwinding case manually.