Add drush myrest:generate and myrest:diff

Problem/Motivation

Generation has to run in a deployment and in CI, not only from an administration page, and it has to be possible to see what a generation would do before it does it — on a site whose API the front end depends on, a blind regeneration is not acceptable.

Proposed resolution

• drush myrest:generate, with options to limit to one bundle or endpoint, and a dry-run mode.
• drush myrest:diff printing what would change: files added, files rewritten, files skipped, conflicts, and configuration entities that would change.
• Exit codes that let CI fail when the committed generated code is out of date with the model — the check that keeps the repository honest.
• No interactive prompts in either, so they work unattended.

Remaining tasks

Everything.