Prior to this commit, delegated types were unable to infer their inverse
associations, so saving records built with the generated
`#build_ASSOCIATION` methods weren't writing to all the necessary
places. For example:

```ruby
entry = Entry.create! entryable: Message.new(subject: "Hello world!")
entry.message.subject # => "Hello world!"

entry.build_entryable(subject: "Goodbye world!").save!
entry.reload.message.subject # => "Hello world!"
```

The fact that the `Entry` test model declared a `delegated_type
:entryable` definition with `types: %w[ Message Comment ]` was never
reciprocated in the appropriate models.

In order to pass the tests, this commit needed to define the
corresponding `has_one :entry` associations. To do so, introduce the
`Entryable` concern in the same style as the one mentioned in the
documentation. The same extraction is made for a `UuidEntryable` concern
mixed into `UuidMessage` and `UuidComment`.

Unfortunately, defining `delegated_type :thing, types: %w[ Post ]` was
more tricky to fix. The `Post` test model is widely used, so defining a
`has_one` had farther-reaching effects than intended. To resolve that
issue, this commit redefines `:thing` to use `types: %w[ Essay ]`, which
has much fewer unintended side effects.