Rely on the state stack to display reduction traces.

Rely on the state stack to display reduction traces.

To display rhs symbols before a reduction, we used information about the rule
reduced, which required the tables yyrhs and yyprhs.  Now use rely only on the
state stack to get the same information.

	* data/lalr1.cc (b4_rhs_data, b4_rhs_state): New.
	Use them.
	(parser::yyrhs_, parser::yyprhs_): Remove.
	(parser::yy_reduce_print_): Use the state stack.