Speed up drawing an exponent whenever the number of random bits returned
by the generator is smaller than the number of bits in the integer type.
The speed-up is most pronounced with `std::ranlux24` on a 64-bit system,
> rfpl-benchmark -n 100000000 -d rfpl -g ranlux24 -- -128 + 129
finishes in about 29 sec CPU time instead of 39 sec.