feat(SetTheory/Game/Impartial): Better iff lemmas

[vihdzp]

This PR does the following:

• make x ≈ 0 and x ‖ 0 the simp-normal forms of x ≤ 0, x ⧏ 0, etc. on impartial games.
• swap the direction of some iff lemmas whose LHS were too specific to ever realistically appear.
• make the game argument implicit in all of these lemmas.
• drive-by golfing.

---

• depends on: [Merged by Bors] - chore(SetTheory/Game/Impartial): golf two proofs #21074
• depends on: [Merged by Bors] - chore(SetTheory/Game/Impartial): miscellaneous cleanup #21007
• depends on: [Merged by Bors] - feat(SetTheory/Game/PGame): rewrite left moves of -x as right moves of x under binders #21109

[github-actions]

PR summary 4d0746a884

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ equiv_zero_iff_forall_leftMoves_fuzzy
+ equiv_zero_iff_forall_rightMoves_fuzzy
+ fuzzy_zero_iff_exists_leftMoves_equiv
+ fuzzy_zero_iff_exists_rightMoves_equiv
+ le_zero_iff_equiv
+ lf_zero_iff_fuzzy
+ zero_le_iff_equiv
+ zero_lf_iff_fuzzy

You can run this locally as follows

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

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh 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).

[vihdzp]

This might have an overly strong discrimination key, should I make it scoped simp?

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - chore(SetTheory/Game/Impartial): golf two proofs #21074
• [Merged by Bors] - chore(SetTheory/Game/Impartial): miscellaneous cleanup #21007
• [Merged by Bors] - feat(SetTheory/Game/PGame): rewrite left moves of -x as right moves of x under binders #21109
  By Dependent Issues (🤖). Happy coding!