[!320] Move get_facedir/get_facemod into accumulation mixin

Merge branch 'feature/put-facedir-facemod-info-on-visitor' into 'master'

ref:extensions/dune-codegen This seems like the right thing to do with mixins
in place.

See merge request [extensions/dune-codegen!320]

  [extensions/dune-codegen!320]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/320