Function bulletproofs::util::sum_of_powers

pub fn sum_of_powers(x: &Scalar, n: usize) -> Scalar

Takes the sum of all the powers of x, up to n If n is a power of 2, it uses the efficient algorithm with 2*lg n multiplications and additions. If n is not a power of 2, it uses the slow algorithm with n multiplications and additions. In the Bulletproofs case, all calls to sum_of_powers should have n as a power of 2.