Fix `is_product_type` using `product_type` interface

[viboes]

is_product_type is defined now as follows

  template <typename T>
  struct is_product_type : is_base_of<product_type_tag, product_type_traits<T>> {};

and force the specialization to inherit from product_type_tag. This is intrusive and don't check all. The definition should probe that the specialization product_type_traits<T> provides the good interface instead, or better yet that the product_type interface is respected.

The advantage of the tag however is that it improves compile time.