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

Time complexity $O(|a|)$