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