Add limited support for aggregate, do, try steps

v0.3.0 adds limited support for aggregate, do, and try steps. Jobs and step hooks (e.g. on_success) now take a StepBox, which is a union type over aggregate, do, try, get, task, and put. This permits support for common usecases such as parallel resource fetching, but does not permit arrangements like an aggregate step with a list of try steps.