code: public static FastStr unsafeOf(String s) {
          int sz = s.length();
          if (sz == 0) return EMPTY_STR;
          char[] buf = bufOf(s);
          return new FastStr(buf, 0, sz);
      }
docs: Construct a FastStr instance from a String instance.
  The FastStr instance will share the char array buf with
  the String instance
  @param s the string instance
  @return A FastStr instance