Allow to pass a callable for field-level hash

[hynek]

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:

def __hash__(self):
    return hash((self.a, custom_hash_for_b(self.b)))

while injecting custom_hash_for_b into eval's globals.

[Tinche]

Don't be a coward and inject it into __hash__ as a parameter default.