deriving FromSexp and default fields

```
import KLR.Util
open KLR.Util(FromSexp)

structure pseudo_core_barrier where
  id : UInt32
  header : Int -- := 5
deriving FromSexp
```

Uncommenting the default field gives

```
Function expected at
  id
but this term has type
  UInt32

Note: Expected a function because this term is being applied to the argument
  5
```

Some bug in handling the default values?