index of the first byte of successive UTF-8-encoded code points in the string,
and the second value, of type <code>rune</code>, will be the value of
the corresponding code point.  If the iteration encounters an invalid
UTF-8 sequence, the second value will be <code>0xFFFD</code>,
the Unicode replacement character, and the next iteration will advance
a single byte in the string.
</li>