chore(Topology/CantorBendixson): minor cleanup

[vihdzp]

CantorBendixson is not a type, and the names in this file shouldn't clash with anything else, so we un-namespace the material in this file.

We also rewrap some lines and perform minor golfs.

---

I haven't added deprecations since the material here is quite new, but I can add them if they're nevertheless wanted.

[github-actions]

PR summary 02a7ea0288

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

## summary with just the declaration names:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh <optional_commit>

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit 02a7ea0).

• +19 new declarations
• −19 removed declarations

-CantorBendixson.isClosed_iteratedDerivedSet
-CantorBendixson.isClosed_perfectKernel
-CantorBendixson.iteratedDerivedSet
-CantorBendixson.iteratedDerivedSet_antitone
-CantorBendixson.iteratedDerivedSet_constant_iff_preperfect
-CantorBendixson.iteratedDerivedSet_limit
-CantorBendixson.iteratedDerivedSet_mem_fixedPoints
-CantorBendixson.iteratedDerivedSet_mono
-CantorBendixson.iteratedDerivedSet_succ
-CantorBendixson.iteratedDerivedSet_zero
-CantorBendixson.mem_fixedPoints_of_iteratedDerivedSet_succ_eq
-CantorBendixson.perfectKernel
-CantorBendixson.perfectKernel_empty
-CantorBendixson.perfectKernel_eq_iteratedDerivedSet_of_mem_fixedPoints
-CantorBendixson.perfectKernel_idem
-CantorBendixson.perfectKernel_mono
-CantorBendixson.perfectKernel_subset
-CantorBendixson.perfectKernel_subset_iteratedDerivedSet
-CantorBendixson.perfect_perfectKernel
+isClosed_iteratedDerivedSet
+isClosed_perfectKernel
+iteratedDerivedSet
+iteratedDerivedSet_antitone
+iteratedDerivedSet_constant_iff_preperfect
+iteratedDerivedSet_limit
+iteratedDerivedSet_mem_fixedPoints
+iteratedDerivedSet_mono
+iteratedDerivedSet_succ
+iteratedDerivedSet_zero
+mem_fixedPoints_of_iteratedDerivedSet_succ_eq
+perfectKernel
+perfectKernel_empty
+perfectKernel_eq_iteratedDerivedSet_of_mem_fixedPoints
+perfectKernel_idem
+perfectKernel_mono
+perfectKernel_subset
+perfectKernel_subset_iteratedDerivedSet
+perfect_perfectKernel

---

No changes to strong technical debt.
No changes to weak technical debt.

Current commit 02a7ea0288
Reference commit dec5b2b780

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.py pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).