feat(data/finset/basic): add finset.filter_eq_self (#12717)

and an epsilon of cleanup

from flt-regular

src/data/finset/basic.lean

@@ -1427,15 +1427,19 @@ ext $ assume a, by simp only [mem_filter, and_false]; refl
 
 variables {p q}
 
+lemma filter_eq_self (s : finset α) :
+  s.filter p = s ↔ ∀ x ∈ s, p x :=
+by simp [finset.ext_iff]
+
 /-- If all elements of a `finset` satisfy the predicate `p`, `s.filter p` is `s`. -/
 @[simp] lemma filter_true_of_mem {s : finset α} (h : ∀ x ∈ s, p x) : s.filter p = s :=
-ext $ λ x, ⟨λ h, (mem_filter.1 h).1, λ hx, mem_filter.2 ⟨hx, h x hx⟩⟩
+(filter_eq_self s).mpr h
 
 /-- If all elements of a `finset` fail to satisfy the predicate `p`, `s.filter p` is `∅`. -/
 lemma filter_false_of_mem {s : finset α} (h : ∀ x ∈ s, ¬ p x) : s.filter p = ∅ :=
 eq_empty_of_forall_not_mem (by simpa)
 
-lemma filter_eq_empty_iff (s : finset α) (p : α → Prop) [decidable_pred p] :
+lemma filter_eq_empty_iff (s : finset α) :
   (s.filter p = ∅) ↔ ∀ x ∈ s, ¬ p x :=
 begin
   refine ⟨_, filter_false_of_mem⟩,
@@ -1457,7 +1461,7 @@ lemma filter_empty : filter p ∅ = ∅ := subset_empty.1 $ filter_subset _ _
 lemma filter_subset_filter {s t : finset α} (h : s ⊆ t) : s.filter p ⊆ t.filter p :=
 assume a ha, mem_filter.2 ⟨h (mem_filter.1 ha).1, (mem_filter.1 ha).2⟩
 
-lemma monotone_filter_left (p : α → Prop) [decidable_pred p] : monotone (filter p) :=
+lemma monotone_filter_left : monotone (filter p) :=
 λ _ _, filter_subset_filter p
 
 lemma monotone_filter_right (s : finset α) ⦃p q : α → Prop⦄