Add iota_iter

[alugowski]

For converting loops like this:

// Sequential
for (int i = 0; i < 100 : ++i) {
    std::cout << i;  // loop body
}

To parallel for_each:

// Parallel
using poolstl::iota_iter;

std::for_each(poolstl::par, iota_iter<int>(0), iota_iter<int>(100), [](auto i) {
    std::cout << i;  // loop body
});