Simplify return operation

object/object.go

@@ -151,14 +151,15 @@ func (s *String) Clone() Object {
 type BuiltinFunction func(args ...Object) Object
 
 type Builtin struct {
-	Fn BuiltinFunction
+	Name string
+	Fn   BuiltinFunction
 }
 
 func (b Builtin) Type() ObjectType {
 	return BUILTIN_OBJ
 }
 func (b Builtin) Inspect() string {
-	return "builtin function"
+	return fmt.Sprintf("<built-in function %s>", b.Name)
 }
 
 type Array struct {