Update IntInf_strop.

The GMP mpz_get_str function uses digits 0..9a..zA..z for a positive
base and uses digits 0..9A..Za..z for a negative base.  The SML Basis
Library IntInf.fmt function requires 0..9A..F digits for
StringCvt.HEX.  Previously, used a positive base for mpz_get_str and
then converted lower-case to upper-case via a for-loop.  Now, simply
use a negative base for mpz_get_str and need only convert a leading
'-' to a leading '~'.