Implement `util::any`

Partial implementation of `std::any` from C++17 standard. See: http://en.cppreference.com/w/cpp/utility/any

The implementation is in the `util` library as `util::any`.

Deviations from the standards description of `std::any`:
  * Does not avoid dynamic allocation of small objects.
  * Does not implement the `in_place_type<T>` constructors from the standard.
  * Does not implement the `emplace` modifier from the standard.