- adds an example "flat" site with, for now, a single index in root for
    testing purposes
  - wrt-render-all handles default entry instead of just using "new"
  - EntryStore::all_renderable() correctly filters out paths ending in
    "/index" instead of "index", which if nothing else corrects a subtle
    bug that would have gotten in the way of naming something "windex"
  - WRT::handle() will return the contents of a root-level "index"

- moves template existence check to new()
  - uses file_get_contents() instead of usual file slurping boilerplate
  - documents overlay option
  - deletes superfluous entry_wrapped()
  - tidies some commments

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.

Moar memory, less potential confusion doing it in place.

Also made it send errors to the log instead of displaying them.