Refine `spec-gen` to import items from previous forks if they have no types that are changing

[ralexstokes]

consider compute_shuffled_index - right now the spec-gen blindly copies this function to each generated spec version, although there is no reason to do so unlike other functions that need to be updated with new types

the spec-gen utility should be updated to only copy something if it needs to be updated, and otherwise just import the existing thing from the previous spec