Avoid obsolescent features in push_iterator

[frederick-vs-ja]

1. std::iterator is deprecated in C++17 (see cppreference and P0174R2). It's better to explicitly provide the member typedef-names and avoid inheritance.
2. _Is_checked_helper became meaningless since VS 2017 15.8/19.15. Removal of it is planned in <xutility>: What's the intent of the pseudo-documentation? microsoft/STL#5081. I think we should stop using it for newer versions of MSVC.