Add Generator class and support code

Generator is intended to be the preferred way to encapsulate Func building in user pipelines. This pull request probably could use more documentation, but the overall design and set of tests is solid enough to begin serious discussion.
Hrm, that's a good point. (The internal-to-Google equivalent may predate some of Target.) Though I'd argue it's still convenient to have as a member method here too, even if it (also) exists as a Target method.