parser: Make debug log for payload substructure rules more clear

parser: Make debug log for payload substructure rules more clear

Instead of this

  parsing rule 10 (1258)

we now see this

  parsing rule 10 LIST of PROPOSAL_SUBSTRUCTURE

in the debug log. Particularly useful if the list is empty as there won't
be a message like this following it:

  x bytes left, parsing recursively PROPOSAL_SUBSTRUCTURE