Work out more details of how json_free() needs to work.

The idea is that you can free any element once you are
done with it, even if it is part of an array or hash that
isn't completely read yet.

Removing array elements is reasonably well defined because
nothing else depends on them. Removing a hash key or value
causes it to be replaced by a null unless the other side of
the component has already been nulled out, in which case
the whole pair is removed.

jsonpull.h

@@ -33,5 +33,6 @@ typedef struct json_pull json_pull;
 json_pull *json_begin_file(FILE *f);
 json_pull *json_begin_string(char *s);
 json_object *json_parse(json_pull *j);
+void json_free(json_object *j);
 
 json_object *json_hash_get(json_object *o, char *s);