docs: public static String join(Iterator strings, String sep) {
          if (!strings.hasNext())
              return "";
  
          String start = strings.next().toString();
          if (!strings.hasNext()) // only one, avoid builder
              return start;
  
          StringBuilder sb = StringUtil.borrowBuilder().append(start);
          while (strings.hasNext()) {
              sb.append(sep);
              sb.append(strings.next());
          }
          return StringUtil.releaseBuilder(sb);
      }