Fix the iterableFactory of mutable Sets

[szeiger]

This points to a soundness bug in the compiler. These errors should be
caught. Omitting a necessary override leads to the expected errors in
some cases but not in all.

[julienrf]

Omitting a necessary override leads to the expected errors in
some cases but not in all.

I hope we can fix that in the compiler :(

[szeiger]

Looks like a similar issue to scala/bug#7093, present in Scala and Dotty, with no solution since it was opened 5 years ago, so I wouldn't hold my breath.

[szeiger]

I could have held my breath after all because Dotty fixed it: scala/scala3#4013. See my comment there. Not only are we exploiting unsound protected[this] in my design changes, we already did before, just as the old collections library does.