A versioned thing, accompanied by its version.
Note that, as used in LinkingUnit, the version is relative to the
identity of the versioned thing. The definition of identity varies as
items progress through the linking pipeline, but it only gets stronger,
i.e., if two items are id-different at phase P, then they must also be
id-different at phase P+1. The converse is not true. This guarantees that
versions can be reliably used to determine at phase P+1 whether the given
item coming from phase P must be reprocessed.