long long support

[William Morriss]

I am using gmpxx and I noticed that long longs, particularly uint64_t, were
not supported, and that the documentation also notes this gap. I am
wondering whether this is because of some real limitation or just missing
functionality. It seems to have been missing for a long time even though
long long was standardized in C++11. I would also consider adding the
support myself.

Thanks,
William Morriss