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.
Should this go in the Target class? I know I'd have liked to have this function in several places where I might not have a generator, but I do have a Target (Target is a much lighter weight 'thing' than a generator).