Extension sets and lists with variables in hugr-model

[zrho]

hugr-model at this point only supports extension sets and lists that have at most one variable, which needs to be at the end in the case of lists. hugr-core allows any number of variables to occur in extension sets and rows. I was initially skeptical that this might make the type theory quite a bit more complicated. I now think that this additional complexity is manageable enough so that I don't have problem with making hugr-model support multiple variables in extension sets and rows. That would fix one of the remaining mismatches.