Standardize Trait Style

[akleeman]

As @jbangelo suggested it'd be nice to switch the trait inspection style from something like this:

template <typename X> class is_complete {
  template <typename T, typename = decltype(!sizeof(T))>
  static std::true_type test(int);
  template <typename T> static std::false_type test(...);

public:
  static constexpr bool value = decltype(test<X>(0))::value;
};

to something like this:

template<typename T, typename = void>
struct is_complete : std::false_type { };

template<typename T>
struct is_complete<T, typename std::enable_if<sizeof(T) != 0>::type> : std::true_type { };