Remove Display impls; Line numbers in errors; Resolve TODOs
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.