User defined REVERSE() function Unicode support 🎉

[teizz]

As discussed with @JanJakes in pull request #434 I've gone ahead and added the code to support unicode characters while keeping behavior aligned with how mysql reverses a utf8mb4 unicode string, i.e. it correctly reverses multi-byte characters, but fails to preserve the correct order for grapheme clusters / combining characters.

As a practical example: both this implementation and mysql's REVERSE() function will reverse '🧟 🧟‍♂️ 🧟‍♀️' to become '♀‍🧟 ️♂‍🧟 🧟' instead of '🧟‍♀️ 🧟‍♂️ 🧟'.

I've also gone ahead and added a check to enable a fast path with strrev() in case all characters are normal ASCII, this improves both speed and prevents additional allocation for arrays.

Finally, I checked to see that

    preg_match_all( '/./u', $str, $matches );
    return implode( '', array_reverse( $matches[0] ) );

is actually about twice as fast as

    $chars = preg_split( '//u', $str, -1, PREG_SPLIT_NO_EMPTY );
    return implode( '', array_reverse( $chars ) );