message: Use insert for faster appending in xsputn
diff: @@ -365,11 +365,7 @@ protected:
  }
  std::streamsize xsputn(const char_type* s, std::streamsize n) override {
  - // TODO: Do a performance analysis whether the current implementation based
  - // on copy_n is faster than these two statements:
  - // (1) container_.resize(container_.size() + n);
  - // (2) std::memcpy(this->pptr(), s, n * sizeof(char_type));
  - std::copy_n(s, n, std::back_inserter(container_));
  + container_.insert(container_.end(), s, s + n);
  return n;
  }
  