Thanks for the review.

I pushed a change to check if clz can be used (`sizeof(digit) <= sizeof(unsigned int)`). Otherwise use clzl. I believe the latter should be the most common, since unsigned long has 32bits. As you say unsigned long long should never be needed.

Btw. mathmodule.c currently duplicates the function: https://github.com/python/cpython/blob/master/Modules/mathmodule.c#L1317. It might be worth factoring it out, but I don't know where to put it.