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).
I like moving all of these cases to one place like this, but doesn't it mean the constructors' types all become `Workflow`, rather than something more specific? Previously, we were taking advantage of those more specific types, but that was probably a symptom of the "WorkflowBuilder" problem, and should be less necessary in the near future. Make sense?