Function sum_of_sum_of_prod

pub fn sum_of_sum_of_prod<T>(a: Vec<T>) -> T
where
    T: Copy + Mul<Output = T> + AddAssign + Zero,

2要素の積の総和

Σ{i = 1 ~ N - 1}Σ{j = i + 1 ~ N} aᵢ * aⱼ

Time complexity $O(|a|)$