STYLE: use spanstream instead of stringstream in more places

- particularly useful in these combinations:

  1.
      OCharStream buf;
      // populate

      ISpanStream is(buf.view());
      // parse

  2.
      // read from file
      ifile.getLine(str);

      ISpanStream is(str);
      // parse

  These avoid making a copy of the character content, compared to
  versions with stringstream:

      OStringStream buf;
      IStringStream is(buf.str());