Expand integral powers of complex nos

The `pow_number` needs further checks for overflow.

This looks good. This assumes that `n>0`. Perhaps you can change it to `unsigned long`, then it's clear. I think it works also for `n=0`, i.e. it returns 1, so I think that's fine. Then we need a similar function, that does it for negative `n` --- essentially by calling this `pow_number` and then dividing by the result.