Function sum_of_sum_of_sum

pub fn sum_of_sum_of_sum<T>(a: Vec<T>) -> T
where
    T: Copy + Add<Output = T> + Zero,

2要素の和の総和

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

Time complexity $O(|a|)$