feat(data/finset/basic): equivalence of finsets from equivalence of types (#4560)

Broken off from #4259.
Given an equivalence `α` to `β`, produce an equivalence between `finset α` and `finset β`, and simp lemmas about it.

Author: b-mehta

src/data/finset/basic.lean

@@ -1412,6 +1412,7 @@ ext $ λ _, by simp only [mem_image, multiset.mem_to_finset, exists_prop, multis
 theorem image_val_of_inj_on (H : ∀x∈s, ∀y∈s, f x = f y → x = y) : (image f s).1 = s.1.map f :=
 multiset.erase_dup_eq_self.2 (nodup_map_on H s.2)
 
+@[simp]
 theorem image_id [decidable_eq α] : s.image id = s :=
 ext $ λ _, by simp only [mem_image, exists_prop, id, exists_eq_right]
 
@@ -2163,6 +2164,24 @@ subrelation.wf H $ inv_image.wf _ $ nat.lt_wf
 
 end finset
 
+namespace equiv
+
+/-- Given an equivalence `α` to `β`, produce an equivalence between `finset α` and `finset β`. -/
+protected def finset_congr (e : α ≃ β) : finset α ≃ finset β :=
+{ to_fun := λ s, s.map e.to_embedding,
+  inv_fun := λ s, s.map e.symm.to_embedding,
+  left_inv := λ s, by simp [finset.map_map],
+  right_inv := λ s, by simp [finset.map_map] }
+
+@[simp] lemma finset_congr_apply (e : α ≃ β) (s : finset α) :
+  e.finset_congr s = s.map e.to_embedding :=
+rfl
+@[simp] lemma finset_congr_symm_apply (e : α ≃ β) (s : finset β) :
+  e.finset_congr.symm s = s.map e.symm.to_embedding :=
+rfl
+
+end equiv
+
 namespace list
 variable [decidable_eq α]