Default method for YASON:ENCODE

I wonder if it would make sense to add a default method like this to YASON:ENCODE:

```lisp
(defmethod yason:encode (o &optional s)
  (yason:with-output (s)
    (yason:encode-object o)))
```

This way, ENCODE will automatically work with any objects that define YASON:ENCODE-SLOTS.