Ensure unstable_remove_if calls the predicate exactly once per element

[mjbeardsley]

There are cases where unstable_remove_if can call the predicate on the converged-upon "partition element" twice:
https://godbolt.org/z/a8W5oWr3T

It should have the same guarantee as remove_if:

https://en.cppreference.com/w/cpp/algorithm/remove

Complexity
Exactly std::distance(first, last) applications of the predicate.

https://en.cppreference.com/w/cpp/algorithm/ranges/remove

Complexity
Exactly N applications of the corresponding predicate and any projection, where N = (last - first), and N-1 move operations at worst.

[ericniebler]

Wow, nice find!