nested tuple support

[andrewcorrigan]

I'd really like to make use of this library, but would need support for nested tuples:

  auto one_t = std::make_tuple(std::make_tuple(1),std::make_tuple(1,1));
  auto two_t = std::make_tuple(std::make_tuple(2),std::make_tuple(2,2));

  auto three_t = nested_tuple_map([](int x, int y){return x + y ;}, one_t, two_t);

  assert(three_t == std::make_tuple(std::make_tuple(3),std::make_tuple(3,3)));