cobol: Divide-and-conquer conversion from binary to packed-decimal.

cobol: Divide-and-conquer conversion from binary to packed-decimal.

The legacy routine for converting a binary integer to a packed-decimal
representaion peeled two digits at a time from the bottom of an _int128 value.
These changes replace that routine with a divide-and-conquer algorithm that
runs about ten times faster.

libgcobol/ChangeLog:

	* libgcobol.cc (int128_to_field): Switch to the new routine.
	* stringbin.cc (packed_from_combined): Implement the new routine.
	(__gg__binary_to_packed): Likewise.
	* stringbin.h (__gg__binary_to_packed): Likewise.