Method iterator

libapp2sys/prebuilt/include/google/protobuf/map.h:1440–1440

   public:
    iterator() : iterator_base(iterator_base::kUnknown) {}
    explicit iterator(const DeprecatedInnerIt& dit)
        : iterator_base(iterator_base::kOld), dit_(dit) {}
    explicit iterator(const InnerIt& it)