[ranges] Fix examples that want to be inside a function body

[jensmaurer]

Wrap inside a function.

Example:

auto ints = view::iota(0) | view::take(10);
auto latter_half = drop_view{ints, 5};
for (auto i : latter_half) {
  cout << i << ' ';                             // prints 5 6 7 8 9
}

[jensmaurer]

Editorial teleconference: Strive to put all examples into functions that need to be, as a general policy. Except where it would distract from the readability of the example.