message: fix unsigned comparison
  Summary:
  compiler warns about this
  probably shouldn't do it :)
diff: @@ -48,7 +48,7 @@ class StaticStringBuilder {
  std::string str() const {
  std::string result;
  result.reserve(m_total_chars);
  - for (int i = 0; i < m_index; ++i) {
  + for (uint32_t i = 0; i < m_index; ++i) {
  result += m_strings[i];
  }
  return result;
  