message: fix rvalue reference usage
  Summary:
  Previously, this was using the copy constructor of std::string, now it's using the move constructor as intended.
  Anything with a name is an lvalue. Source:
diff: @@ -38,7 +38,7 @@ class StaticStringBuilder {
  StaticStringBuilder& operator<<(std::string&& s) {
  always_assert(m_index < num_strings);
  m_total_chars += s.size();
  - m_strings[m_index] = s;
  + m_strings[m_index] = std::move(s);
  ++m_index;
  return *this;
  }
  @@ -67,7 +67,7 @@ class DynamicStringBuilder {
  // Take ownership of s
  DynamicStringBuilder& operator<<(std::string&& s) {
  m_total_chars += s.size();
  - m_strings.push_back(s);
  + m_strings.push_back(std::move(s));
  return *this;
  }
  