R1:ec3964944944

Minor tidying of `DivinerWorkflow` classes

Summary: Minor tidying and modernizing a few things.

Test Plan: Ran `./bin/diviner atomize` and `./bin/diviner generate`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11587