Method push_front

include/ilist.h:210–210

  void pop_back() noexcept { erase(--end()); }

  void push_front(reference value) noexcept { insert(begin(), value); }
  void pop_front() noexcept { erase(begin()); }

  // STL version is O(n) but this is O(1) because an element can't be inserted