Code restructuring, in preparation for further work.

* ot/libify-get-ref-atom-value:
  ref-filter: libify get_ref_atom_value()
  ref-filter: add return value to parsers
  ref-filter: change parsing function error handling
  ref-filter: add return value && strbuf to handlers
  ref-filter: start adding strbufs with errors
  ref-filter: add shortcut to work with strbufs