Fix walking over Pandoc JSON trees

The JSON tree structure has changed from

    {'t': <type>, 'c': <content>}

to

    {<type>: <content>}

so the walk implementation had to be adjusted. Also, 'unMeta' does
not seem to be a mandatory field of tree elements, so this commit
includes a fix to fall back to None.
Fix element constructors for the new {<type>: <content>} syntax