For more information on ICU's Collator and how to
tune it up in stringi, refer to
stri_opts_collator.Uses a stable sort algorithm (STL's stable_sort);
performs up to $N*log^2(N)$ element comparisons, where
$N$ is the length of str.
Interestingly, stri_order is most often faster that
R's order.
Missing values are always put at the end of a character
vector.
stri_sort is a `black sheep` in stringi: it
does not always return UTF-8-encoded strings. Moreover, it
preserves many input object's attributes. This is because
it is defined as str[stri_order(str, decreasing,
opts_collator)].