Freezing unfrozen dataclasses requires unfreezing them beforehand.

```python
import dataclasses
from etils import edc
@edc.dataclass(allow_unfrozen=True)
@dataclasses.dataclass(eq=True, kw_only=True)
class Conf:
x: int = 3

a = Conf()
a.x = 2 # verify that a is not frozen
a.frozen() # complains that .frozen() can only be called after .unfrozen()
```

As the code above shows, `allow_unfrozen` seems to assume that the dataclass is frozen by default. I think that it should verify the assumption.