This entails a bunch of stuff, and I'm not sure how good an idea a lot of
it is.

  - Adds a new tagging format for properties:

      tag.foo.bar.prop

    Where `foo` and `bar` are treated as elements of a path.  Thus if
    `archives/2019/10/5/tag.topics.technical.prop` exists, then
    the entry at `/topics/technical` will link back to `/2019/10/5`.

  - Adds "virtual" entries - if tags link to entries that don't exist,
    they're implicitly created.

  - Adds a concept of metadata distinct from (but about) entry text, and
    caches this for every entry.

  - Adds caching of child entries to EntryStore.

  - Migrates `get_sub_entries()` and `subentry_expr` into EntryStore.

There are XXX comments littered all over the place at this point.  Stuff
will definitely change before a release.

- give absolute paths to imgsize() so it chills out on Cwd::getcwd() calls
- rip out App::WRT::MethodSpit's convenience accessors and just access
  elements of $self directly
- kill local_path()
- stop using a() in entry_markup()
- cache get_date_entries_by_depth() results
- swap out state vars for stashing things on $self in
  get_all_source_files()

This gets rendering the whole p1k3 repo back down in the neighborhood of
2.1 seconds, which is a lot more tolerable than 5 or 6.

It also kinda technically breaks the API, maybe, so I guess this'll be a
v5.0.0 after a while.

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.

Sometimes it becomes painfully obvious that you never ran the tests.

print a(
    {
      title => 'previous month',
      href  => '/2008/2'
    },
    'go to the previous month'
  );

Which should give

  <a title="previous month" href="/2008/2">go to the previous month</a>

Or something very similar.  I suppose there are no guarantees about the
order of attributes.

entry.