fontc: improve the 'work creation' pattern

- how work creation works is not very well documented or obvious.
- there are a few places where I think a trait could simplify things significantly. Mainly It feels like all of the `add_x_job` methods want to be generic, so that you just have something like Workflow::add_job, and this calls a trait method like Work::create.
- the add_x_job implementations are kind of opaque and it's not clear what you're supposed to do here. Another benefit of a trait is that it gives you somewhere to document what is expected.