Applies the given selector to all elements in the given collection and calculates the sum of the results

Example:

import { sumOf } from "https://deno.land/std@0.116.0/collections/mod.ts"
import { assertEquals } from "https://deno.land/std@0.116.0/testing/asserts.ts"

const people = [
    { name: 'Anna', age: 34 },
    { name: 'Kim', age: 42 },
    { name: 'John', age: 23 },
]
const totalAge = sumOf(people, i => i.age)

assertEquals(totalAge, 99)