code: public static FastStr of(StringBuilder sb) {
          int sz = sb.length();
          if (0 == sz) return EMPTY_STR;
          char[] buf = new char[sz];
          for (int i = 0; i < sz; ++i) {
              buf[i] = sb.charAt(i);
          }
          return new FastStr(buf, 0, sz);
      }
docs: Construct a FastStr from a StringBuilder
  @param sb the string builder
  @return a FastStr