Allow to pass a callable for field-level hash

Apparently, being able to customize the hashing of individual fields is something interesting for NumPy users: https://stackoverflow.com/q/74975393/476759

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I guess the idea would be to create a `__hash__` that looks like this:

```python
def __hash__(self):
    return hash((self.a, custom_hash_for_b(self.b)))
```

while injecting `custom_hash_for_b` into `eval`'s globals.