Asymmetry: `∈` replaced by `in` but `∉` stays the same

[fredrikekre]

Right now ∈ is replaced by in but since ∉ exists the formatted code may become
asymmetric since ∉ is left alone and not replaced by e.g. !(...). Perhaps ∈ should be
left alone as well for now.

[fredrikekre]

Actually, I have only implemented the replacement in loop contexts so e.g. x ∈ X stays the same. Since ∉ can't be used in loop contexts I think it is fine as is.