Change Mongo’s PhysicalPlan from WFTask to WFOp.

This gets us one step closer to isolating MongoDB-specifics from the
rest of the system.

It has the nice benefit of cleaning up many tests and making them a bit
more resistant to changes in `physical.mongodb`. Also, one evaluator
test (expecting failure) is now impossible to hit, since we can’t get a
badly-formed `FoldLeftOp` to the evaluator.

However, coverage has dropped, since `WorkflowOp.finalize` and `.crush`
are no longer tested by the planner tests.

Fixes #366.