Feature: Implement ucl_iterate_object_end

[kevemueller]

In a codebase heavily relying on UCL (https://github.com/freebsd/pkg) there is a recurring programming error causing a memory leak involving ucl_iterate_object.

When the iteration is abandoned in-flight, the iterator itself is never freed.
E.g. (abbreviated)

	ucl_object_iter_t it = NULL;
	while ((cur = ucl_iterate_object(obj, &it, true))) {
		if (error) {
			return (EPKG_FATAL); // it is leaked here
		}
	}

Not in every context is it possible to introduce UCL_FREE (sizeof (*it), it);, also it is not evident that that this is the only thing that needs to be done with the iterator.

I suggest introducing ucl_iterate_object_end(ucl_object_iter_t *iter) that can be called before abandoning iteration which does whatever is needed to be done to close the iterator and free all its resources.