Converting between bignums and unsigned is useful even when there's no

support for INT64 (which otoh do seem a bit rare in this day and age), so
changed the uint64 functions to ulongest.

Rev: src/bignum.c:1.44
Rev: src/bignum.h:1.33
Rev: src/modules/Gmp/mpz_glue.c:1.176
Rev: src/modules/Gmp/my_gmp.h:1.26