Workflow: take `history` off of the inner workflow JSON

[josephjclark]

The Workflow class currently maintains an internal JSON representation of the workflow. This is mostly for legacy reasons - when we introduced the Workflow class, we wanted easy compatibility in tests with the old workflow structure.

The history implementation writes directly to Workflow.workflow (or this.workflow). But I don't think it should - history should be a first-class Workflow field.

We may need to modify the toJSON call to ensure history gets written to the serialized workflow

We probably also want to remove that internal representation because it is confusing