- move get_date() from App::WRT::Util to App::WRT::Date
- make App::WRT::FileIO and ::Util less redundant
- add App::WRT::Mock::FileIO for faking writes in tests
- make bin/wrt-render-all testable, add t/bin-wrt-render-all.t

I never could quite figure out why non-ASCII characters were getting
badly munged in XML::Atom::SimpleFeed output.  Using Encode::decode() on
the content of entries turns out to fix this, although it's probably the
tip of an iceberg.

Already have a file_put_contents(); might as well.

I went and read:

https://pause.perl.org/pause/authenquery?ACTION=pause_namingmodules

Realized I should probably not name this something with an inscrutable
top-level namespace.  WRT is still inscrutable, I guess, but at least
it's now under App.

- Renames all modules, fixes tests (such as they are)
- Zaps old t/validate/ stuff

It's shorter and the letters are all right there on the keyboard.