Categorical hash only hashes underlying physical

[mcrumiller]

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Issue description

Hashes of categoricals only hash the underlying physical. Unless created with a global string cache, all categorical Series have physicals 0...n, and so will hash equivalently. This is probably not great behavior.

import polars as pl

pl.Series(["a"], dtype=pl.Categorical).hash() == pl.Series(["b"], dtype=pl.Categorical).hash()
# True

Installed versions

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