Better translator support for recursion under type constructors

The translator currently only supports recursion under type constructors for a few hard-coded types `pair`, `option` and `list`.

For a minimal example, consider these types:

```
Datatype:
container = Container 'a
End

Datatype:
thing = NonThing | Thing (thing container)
End

Datatype:
thing2 = NonThing2 | Thing2 (thing2 option)
End
```

`register_type` accepts `thing2`, but not `thing`.

The first place where it fails is a `tDefine` somewhere deep in `define_ref_inv`, where unless the type constructor is in {`pair`,`option`,`list`}, the recursive invocation of `THING_TYPE` is not eta-expanded properly before being passed to `tDefine`.