* src/gram.h, src/gram.c (rules_swap): New.

* src/gram.h, src/gram.c (rules_swap): New.
(ritem_longest_rhs): Use it.
* src/gram.h (rule_t): `number' is a new member.
* src/reader.c (packgram): Set it.
* src/reduce.c (reduce_grammar_tables): Move the useless rules at
the end of `rules', and count them out of `nrules'.
(reduce_output, dump_grammar): Adjust.
* src/print.c (print_grammar): It is no longer needed to check for
the usefulness of a rule, as useless rules are beyond `nrules + 1'.
* tests/reduce.at (Reduced Automaton): New test.