The problem with this is that then we have to modify a lot of
lines from, let's say, instance->used, instance->size, etc...
to instance->hdr.used, instance->hdr.size, and so on...
Great, let's do that and get it right please.
This code churn is avoided if we use the struct_group() helper.
It's not "churn" if it is "fix the code to be correct" :)