code: private UTF8String copyUTF8String(int start, int end) {
      int len = end - start + 1;
      byte[] newBytes = new byte[len];
      copyMemory(base, offset + start, newBytes, BYTE_ARRAY_OFFSET, len);
      return UTF8String.fromBytes(newBytes);
    }
docs: Copy the bytes from the current UTF8String, and make a new UTF8String.
  @param start the start position of the current UTF8String in bytes.
  @param end the end position of the current UTF8String in bytes.
  @return a new UTF8String in the position of [start, end] of current UTF8String bytes.