push: rework the bundle2partsgenerators logic Instead of a single list of functions, we now have a list of names and a mapping of names to functions. This simplifies wrapping of steps from extensions. In the same move, declaration becomes decorator-based (syntax sugar, nom nom nom!).

push: move common heads computation into pushop Now that both options (push succeed or fall back) live in pushop, we can move the common heads computation there too. It is a very commonly accessed attribute so it makes a lot of sense to have it in pushop.