Make user-exposed sorts stable, by storing the position of elements
in the original array, and using those positions as a fallback
comparison criterion. The base sort is still hybrid q/insert.
The use of true/false comparison functions is deprecated (but still
supported) and should be replaced by -1/0/1 comparison functions,
driven by the <=> operator.
RFC: https://wiki.php.net/rfc/stable_sorting
Closes GH-5236.
