templater: fix cbor() filter to recursively convert smartset to list
The previous attempt, e3e44e6e7245 "templater: fix cbor() filter to accept
smartset", was incomplete since obj may be a collection containing a smartset.
This works around the problem by converting smartsets recursively. Another
option is to teach cborutil how to encode a smartset. That should be okay,
but I hesitated to add "import smartset" to cborutil.py as the cborutil is