darray: some changes for convenience

darray: some changes for convenience

- Make darray_free also initialize the array back to an empty state, and
  stop worrying about it everywhere.

- Add darray_mem, to access the underlying memory, which we do manually
  now using &darray_item(arr, 0). This makes a bit more clear when we
  actually mean to take the address of a specific item.

- Add darray_copy, to make a deep copy of a darray.

- Add darray_same, to test whether two darrays have the same underlying
  memory (e.g. if the struct itself was value copied). This should used
  where previously two arrays were compared for pointer equality.

Signed-off-by: Ran Benita <ran234@gmail.com>