This is necessary since the implementation of the job functions
calls through the VTable of the interface JobClosure. Thus this
interface (and the VTable definition) needs to reside within
some compilation unit linked together with the basic job class.
TODO: move class Job entirely into the Backend
without any trickery, we'll always get two indirections.
Thus, the decision is to turn the 2nd indirection
into a VTable call; this way, basically the JobClosure
also acts as job functor itself.
