Update erf::polynomial

[chris-ha458]

I changed erf::polynomial into more idiomatic code.

Although tests (cargo test) pass, I wasnt sure about the coverage so I tested it with my own version of the code.

#[cfg(test)]
mod tests {
    use super::*;

    fn run_tests(f: fn(f64, &[f64]) -> f64) -> Vec<(f64, f64)> {
        vec![
            (2.0, vec![1.0, 2.0, 3.0], 17.0),
            (2.0, vec![], 0.0),
            (2.0, vec![5.0], 5.0),
            (
                2.0,
                vec![1000000000.0, 2000000000.0, 3000000000.0],
                17000000000.0,
            ),
            (2.0, vec![0.001, 0.002, 0.003], 0.017),
            (2.0, vec![-1.0, -2.0, -3.0], -17.0),
        ]
        .into_iter()
        .map(|(z, coeff, expected)| {
            let result = f(z, &coeff);
            (result, expected)
        })
        .collect()
    }

    #[test]
    fn test_polynomial() {
        for (result, expected) in run_tests(polynomial) {
            assert_eq!(result, expected);
        }
    }

    #[test]
    fn test_polynom() {
        for (result, expected) in run_tests(polynom) {
            assert_eq!(result, expected);
        }
    }

    #[test]
    fn test_equivalence() {
        for (z, coeff, _) in vec![
            (2.0, vec![1.0, 2.0, 3.0], 17.0),
            (2.0, vec![], 0.0),
            (2.0, vec![5.0], 5.0),
            (
                2.0,
                vec![1000000000.0, 2000000000.0, 3000000000.0],
                17000000000.0,
            ),
            (2.0, vec![0.001, 0.002, 0.003], 0.017),
            (2.0, vec![-1.0, -2.0, -3.0], -17.0),
        ] {
            let result_poly = polynomial(z, &coeff);
            let result_polynom = polynom(z, &coeff);
            assert_eq!(result_poly, result_polynom);
        }
    }
}

The above code tests each the oroginal code polynomial, changed code polynom and whether they are equivalent to each other.