Several peripheral API changes here ramping up to the 0.4.0 release.
- The `All::remove_by_*` functions now return booleans indicating if the
  gruser was removed or not.
- User and Group no longer implement display for use in passwd/shadow
  file serialization; the implementations have moved to private
  functions.
- Parse errors now return a line number (only the line number! Don't
  leak the file contents).
- Invalid lines are no longer ignored during passwd/group file parsing.
  This has potential to cause breakage but I'd estimate that to be a
  pretty small risk. It does now report any parse errors.