MSVC warning C4365: signed/unsigned mismatch in cxx.cc operator<< overloads

[wravery]

After a recent compiler upgrade on Windows, I started seeing a warning that there's a signed/unsigned mismatch in cxx.cc:

std::ostream &operator<<(std::ostream &os, const String &s) {
  os.write(s.data(), s.size());
  return os;
}

The s.size() method returns std::size_t, but os.write takes a signed std::streamsize

I think the same probably applies to the equivalent Str operator below that, but in my case the warning was treated as an error, and it seems to have stopped compilation rather than reporting that one.