add for_each overloads with callables that accept index

[sergio-eld]

This should be supported:

eld::for_each(pod, [](auto &elem, size_t indx){});
eld::for_each(pod, []<size_t Indx>(auto &elem, std::integral_constant<size_t, Indx>){});
eld::for_each(pod, [](auto &elem, const auto& indx){}); // index is of type std::integral_constant<size_t, Indx>