Enhance bench.pl.

Enhance bench.pl.

	* etc/bench.pl.in (parse, parse_expr, parse_term, parse_fact)
	(@token, $grammar, $bench): New.
	(generate_grammar_variant): Rename as...
	(generate_grammar_list): this.
	(generate_grammar): Adjust.
	(bench_grammar): Rename as...
	(bench): this.
	Use it in the various bench-marking routines.
	(-b, -g): New options.