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.
Always duplicate the array before doing a sort with user-defined
comparison function, to avoid access to the intermediate
inconsistent state.
I've also dropped the "array modification" warning, as protection
against modifications is no longer relevant if we're always working
on a copy anyway.
This also required some changes to how SplArray forwards calls to
sorting functions.
Nikita Popov