Investigate specialised mulmod for base*cuml in modexp

[unzvfu]

In the modular exponentiation algorithms, there are repeated mulmods of the form A <- A.g where g is the base and A is the intermediate value of the modexp. Since g is fixed for over all such mulmods, it might be possible to find a better representation of g and an associated specialised mulmod algorithm to speed up those mulmods.

[unzvfu]

Follow up at https://github.com/unzvfu/cuda-fixnum/issues/35.