Denotation string representation

Just as a reminder to do a normal `toString` for denotations. While testing scarph I get test failures often and want to see what are the differences in the values I've got, but instead I see
```
Expected ohnosequences.cosas.types$Denotes@7b9e57cd, but got ohnosequences.cosas.types$Denotes@b3816a07
```