refactor(archive/100-theorems-list/02_cubing_a_cube): review API (#16898)
refactor(archive/100-theorems-list/02_cubing_a_cube): review API (#16898)

* reuse lemmas about `set.pi`;
* fix some `def`/`lemma`;
* use `nontrivial` and `set.nontrivial` instead of `2 ≤ cardinal.mk ι`;
* redefine `correct` as a `structure`.





leanprover-community/mathlib,leanprover-community/mathlib