The `Review::summary` was limited in behaviour, since it was only an
`Option<String>`.
The field is now changed to `NonEmpty<Edit>` so that it can enable an edit
history, as well as being able to supply embeds as part of the text.
`Reactions` are also introduced to `Review` and, necessarily, introducing an
`Action::ReviewReact`.
Note that some machinery was added under `actions` and `encoding` to enable
backwards-compatibility.
For `actions`, a new `ReviewEdit` is introduced to keep
track of the different versions of editing actions – supporting the old actions
while also introducing the new variant.
For `encoding`, the machinery exploits the fact that an `Option<String>` can be
converted into a `NonEmpty<Edit>` given that an `ActorId` and `Timestamp` are
available, and that `None` can be converted into an empty string. The struct
`patch::Review` now deserializes via this `encoding::review::Review` type and,
importantly, preserves backwards-compatibility.
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