unsigned long long conversion

[keith.briggs at bt.com]

> I don't understand your question. It this part of your code is time critical, perhaps you could try different variants and time them.
Yes, it is time-critical, which is why I am considering a low-level implementation.
mul_i.h contains macros and a skeleton for mpz_t * (a scalar integer type), so it should be easy
to add a new function at this point, and presumably quite efficient.
Keith