Improve handling of ExprLabel/Name/Var.

Since `value` is a special name in MongoDB (in that it’s created during
any map/reduce), and our usage of Expr* is often (but not always) to
deal with this, it should be exported from `WorkflowOp`, rather than
redefined in `WorkflowBuilder`.
Add IdLabel/Name/Var to parallel Expr*.

Since, again, it’s a special name as far as Mongo is concerned.