Close the gap between `hugr-model` terms and the types of `hugr-core`.

[zrho]

There still exists a mismatch between what hugr-model terms on the one hand and the Type, TypeArg, TypeParam, TypeRow, etc machinery in hugr-core on the other can express. To ease conversion, to enable features and ideally to enable a gradual move to a uniform term representation in hugr-core (see #2266), it makes sense to close the gap as much as possible.