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).
Had a thought about this. We could capture most of these params in a case class, so you would only have to deal with, say, `src`, `near`, `distanceField` (the non-optinal params), and then a `params` object. That would reduce the silly nine-`_` matching to just four.

I don't know, it's just an idea. I'm still up for ripping it out until we actually have a test. Alternatively, write a test, but I suppose we'll need a `Near` func.