Including:

  - children() - get immediate children of entry
  - parent($entry) - get immediate parent of entry
  - days_for($container) - get days in a month or year
  - months_for($year) - get months contained in a year
  - basename($entry) - get basename of an entry

These are used in App::WRT::year() and App::WRT::month(), replacing
some dir_list() invocations and at least somewhat simplifying the code.