Add new method ucl_array_merge

Merges the contents of an array into an existing array
I dislike this iteration. Since this is an internal API, I would just do the following:

1. Check that `elt` is an array (and the current code lacks this check).
2. Directly iterate `elt` using `LL_FOREACH_SAFE` and steal all elements from it as well.
3. Unref `elt` itself, since all elements are moved to the `top`.

Warn user that this function will anyway move all elements from `elt` to the `top`.