Simplify h_evals loop in verifier

[ebfull]

let (_, h_eval) = self
            .h_evals
            .iter()
            .fold((C::Scalar::one(), C::Scalar::zero()), |(cur, acc), eval| {
                (cur * &x_3n, acc + &(cur * eval))
            });

Rewriting x^n as x, we can apply Horner's rule to simplify

let h_eval = self
            .h_evals
            .iter()
            .rev()
            .fold(C::Scalar::zero(), |acc, eval| {
                acc * &x_3n + eval
            });