chore(library/init/data): add missing instances

library/init/data/repr.lean

@@ -125,6 +125,9 @@ else "\"" ++ string.quote_aux s.to_list ++ "\""
 instance : has_repr string :=
 ⟨string.quote⟩
 
+instance : has_repr string.iterator :=
+⟨λ it, it.next_to_string.quote ++ ".mk_iterator"⟩
+
 instance (n : nat) : has_repr (fin n) :=
 ⟨λ f, repr (fin.val f)⟩
 

library/init/data/to_string.lean

@@ -20,6 +20,9 @@ has_to_string.to_string
 instance : has_to_string string :=
 ⟨λ s, s⟩
 
+instance : has_to_string string.iterator :=
+⟨λ it, it.next_to_string⟩
+
 instance : has_to_string bool :=
 ⟨λ b, cond b "tt" "ff"⟩