chore(logic/function/basic): don't unfold set in cantor (#13822)
chore(logic/function/basic): don't unfold set in cantor (#13822)

This uses `set_of` and `mem` consistently instead of using application everywhere, since `f` has type `A -> set A` instead of `A -> A -> Prop`. (Arguably, it could just be stated for `A -> A -> Prop` instead though.)



leanprover-community/mathlib,leanprover-community/mathlib