Convert WorkflowOp to Fixplate.

This works, but there are likely some places that could be cleaned up (EG, switching between `Term[WorkflowOp]` and `WorkflowOp[Term[WorkflowOp]]` to minimize instances of `Term()` and `unFix`) and possibly additional opportunities to take advantage of fixed-point morphisms.

Much smaller than the original WorkflowOp merge, though.