fix(data/setoid/partition): make def more readable (#8951)
fix(data/setoid/partition): make def more readable (#8951)

If we change the statement of `partition.order_iso` from `setoid α ≃o subtype (@is_partition α)` to `setoid α ≃o {C : set (set α) // is_partition C}` then this doesn't change anything up to defeq and it's much easier for a beginner to read, as well as avoiding the `@`. I also change some variable names. Why? I want to show this part of this file to undergraduates and I want to make it look as easy and nice as possible.





leanprover-community/mathlib,leanprover-community/mathlib