* src/derives.h, src/derives.c (derives): A `rule_t***' instead of

* src/derives.h, src/derives.c (derives): A `rule_t***' instead of
`rule_number_t**'.
(set_derives, free_derives): Rename as...
(derives_compute, derives_free): this.
Adjust all dependencies.
* src/nullable.c (set_nullable, free_nullable): Rename as...
(nullable_compute, nullable_free): these.
(rule_list_t): Store rule_t *, not rule_number_t.
* src/state.c (state_rule_lookaheads_print): Directly compare rule
pointers, instead of their numbers.
* src/main.c (main): Call nullable_free, and derives_free earlier,
as they were lo longer used.