rpst_iterate_first
for (rn = rpst_iterate_first(&t, max_y, min_x, max_x, &it);
assert(NULL == rpst_iterate_first(&t, UINT64_MAX, 0, UINT64_MAX, &it));
struct rpst_node *rpst_iterate_first(struct rpst_tree *, uint64_t, uint64_t,