Code duplication: For and Macro

[liamhuber]

In #373, the missing behaviour for for-loops to reload the value-links between their input and their children is added. Right now it's by straight-up code duplication from the Macro implementation. Instead this should come from a shared ancestor, e.g. StaticComposite in analogy to StaticNode. Right now that results in a circular import because Composite holds an instance of Creator; in the long run, Workflow should be better extracted from the node hierarchy, it should be the only one to hold the Creator, and I'll be able to introduce such a shared ancestor for these too.