ptr_to_unique<T>, extending std::unique_ptr to support weak secondary smart pointers

ptr_to_unique is a smart pointer to an object already owned by a unique_ptr that is guaranteed to read as null if the object has been deleted ensuring that it never dangles. std::unique_ptr is extended to support this by exploiting its provision for custom deleters to allow secondary smart pointers to be informed of deletions. The implementation uses a hidden reference counted control block similar to that of shared_ptr/weak_ptr but more lightweight and tailored to the requirements of single ownership.