cobol: Implement faster zoned decimal to binary conversion.

cobol: Implement faster zoned decimal to binary conversion.

Replace " value *= 10; value += digit" routines with a new one that does two
digits at a time and avoids __int128 calculations until they are necessary.

These changes also clean up the conversion behavior when a digit is not valid.

gcc/cobol/ChangeLog:

	* genutil.cc (get_binary_value): Use the new routine.

libgcobol/ChangeLog:

	* libgcobol.cc (int128_to_field): Use the new routine.
	(get_binary_value_local): Use the new routine.
	(format_for_display_internal): Formatting.
	(__gg__get_file_descriptor): Likewise.
	* stringbin.cc (string_from_combined): Formatting.
	(packed_from_combined): Likewise.
	(int_from_string): New routine.
	(__gg__numeric_display_to_binary): Likewise.
	* stringbin.h (__gg__numeric_display_to_binary): Likewise.