code: public static FastStr unsafeOf(char[] buf, int start, int end) {
          E.NPE(buf);
          E.illegalArgumentIf(start < 0 || end > buf.length);
          if (end < start) return EMPTY_STR;
          return new FastStr(buf, start, end);
      }
docs: Construct a FastStr instance from char array, from the start position, finished at end position
  without copying the array. This method might use the array directly instead of copying elements
  from the array. Thus it is extremely important that the array buf passed in will NOT be updated
  outside the FastStr instance.
  @param buf the char array
  @param start the start position (inclusive)
  @param end the end position (exclusive)
  @return a FastStr instance that consist of chars specified