`Message` is an enum with multiple variants. Four of those variants map
directly onto the four variants of `WorkItemResult`. This commit reduces
those four `Message` variants to a single variant containing a
`WorkItemResult`. This requires increasing `WorkItemResult`'s visibility
to `pub(crate)` visibility, but `WorkItem` and `Message` can also have
their visibility reduced to `pub(crate)`.
This change avoids some boilerplate enum translation code, and makes
`Message` easier to understand.
88cd8f9324