Introduce a hierarchy for symbols.

Introduce a hierarchy for symbols.

	* data/lalr1.cc (symbol_base_type, symbol_type): New.
	(data_type): Rename as...
	(stack_symbol_type): this.
	Derive from symbol_base_type.
	(yy_symbol_value_print_): Merge into...
	(yy_symbol_print_): this.
	Rename as...
	(yy_print_): this.
	(yydestruct_): Rename as...
	(yy_destroy_): this.
	(b4_symbols_actions, YY_SYMBOL_PRINT): Adjust.
	(parser::parse): yyla is now of symbol_type.
	Use its type member instead of yytoken.