about unpack_sequence customization point

I'm not completely against the way you have customized unpack, but I prefer to overload non-member functions than using some kind of traits.

I would preferred if `std::experimental::apply` could be customizable and so

```
unpack(f)(product_type) <==> std::tuple::apply(f, product_type)
```

This has the advantage that the user can use the `apply` function when it has the function and the product-type.

What about replacing `unpack_sequence` by an overloaded `apply`?

Overloading `apply` has the liability that it is not a high-order function, but we can have an `apply_fn` that is hof.