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 $\sum_{i = 1}^{N - 1} \sum_{j = i + 1}^N a_i + a_j$

Time complexity $O(|a|)$