Migrate Composites' zip machinery
This migrates the v2::Composites container to use the machinery introduced in !2811 (merged) and fixes the fallout.
Because not all containers are migrated (see #103, though migrating to SOACollection is not sufficient) this means introducing some helpers for uniformly using the two sets of zip machinery. This is necessary because some algorithms (e.g. ThOr::Combiner have handle inputs that use a mix of the two machineries).
Follow-up: remove the compatibility layer when it is no longer needed.
Goes with Rec!2273 (merged) and Phys!827 (merged).