chore: annotate OfNat and ToString default instances

src/Init/Data/ToString/Basic.lean

@@ -25,6 +25,7 @@ instance {α} [ToString α] : ToString (id α) :=
 instance {α} [ToString α] : ToString (Id α) :=
   inferInstanceAs (ToString α)
 
+@[defaultInstance]
 instance : ToString String :=
   ⟨fun s => s⟩
 

src/Init/Prelude.lean

@@ -345,6 +345,7 @@ class OfNat (α : Type u) :=
 
 export OfNat (ofNat)
 
+@[defaultInstance]
 instance : OfNat Nat := ⟨id⟩
 
 instance : Inhabited Nat := {

tests/lean/run/meta7.lean

@@ -167,3 +167,11 @@ def tst8 : MetaM Unit := do
   pure ()
 
 #eval tst8
+
+def tst9 : MetaM Unit := do
+  print "----- tst9 -----"
+  let defInsts ← getDefaultInstances `OfNat
+  print (toString defInsts)
+  pure ()
+
+#eval tst9