Convert WorkflowOp to Fixplate.

Partial fix for #35.

The main practical benefit at the moment is that it allows us to unify
Workflows and Pipelines.

There is a bunch of renaming:

* WorkflowOp                           → WorkflowF
* Term[WorkflowOp]                     → Workflow
* Term[WorkflowOp] => Term[WorkflowOp] → WorkflowOp
* WorkflowOp.workflow                  → Workflow.task
* someOp                               → $some

Also,

* `Workflow` case class constructors are now non-private, for the `Traverse`
  impl as well as for `deleteUnusedFields` and `rewrite` (which currently needs
  to return the same type that was given to it, not a Term) and
* IdHandling is a bit more strict (but makes one property test hard
  to implement correctly, so it’s marked pending).
This case is less clear-cut than, say, `crush`, where we actually use a paramorphism, but I originally (in the previous fixplating attempt) used `cata` here. However, now that it’s being called in the constructors, it doesn’t need to make the recursive calls anymore and could still be separate methods, but I do think trying to avoid caring about the sum types is a good thing in general.