optimizing `build_array`

Check whether we can replace a naive implementation of computing [g^0; g^1; ..; g^{n-1}] with a more optimized one (if squaring is more efficient than multiplication):

res = [g^0; g^1; ..; g^{n-1}], i.e., res[i] = g^i

if n is divisible by 2, res can be computed as follows

res[0] = one
res[1] = g

for (int i = 1; i < n / 2; i++) {
  res[2*i] = res[i] * res[i];
  res[2*i+1] = res[2*i] * g;
}