feat: FoldAdd and FoldMul

[Daniel-Aaron-Bloom]

These traits make it really easy and readable to do a bunch of sums or products, especially on generics:

fn foo<A, B, C, D>(_: FoldSum::<tarr![A, B, C, D]>)
where tarr![A, B, C, D]: FoldAdd {
    /*...*/
}

Compared to:

fn foo<A, B, C, D>(_: Sum<Sum<Sum<A, B>, C>, D>)
where A: Add<B>,
    Sum<A, B>: Add<C>,
    Sum<Sum<A, B>, C>: Add<D> {
    /*...*/
}