Make sure we correctly forbid recursive datatypes that we don't want to support

Ensure we give a nice error message that explains that recursive types are forbidden

```
Type error:
[struct_rec.sail]:3:10-11
3 |  field : S
  |          ^
  | Undefined type S
  | This error was caused by:
  | [struct_rec.sail]:2:0-4:1
  | 2 |struct S = {
  |   |^-----------
  | 4 |}
  |   |^
  |   | Recursive types are not allowed
```

The theorem prover backends create a special register_value union that
can be recursive, so we make sure to special case that.

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