Hash frozensets deterministically

[sarathfrancis90]

While looking at how transforms get fingerprinted I noticed frozenset isn't hashed deterministically, unlike set.

set has a custom reducer (_save_set) that pickles its elements sorted, so the fingerprint is stable across runs. frozenset has no such reducer and falls back to the default one, which keeps the iteration order. Because Python randomizes the string hash seed per process, a transform that captures a frozenset of strings gets a different fingerprint every session, so the cache never hits.

Repro:

from datasets.fingerprint import Hasher
# prints a different hash on every run (PYTHONHASHSEED is random by default)
print(Hasher.hash(frozenset("abcdefghij")))

I registered the same sorted reducer for frozenset that set already uses (falling back to sorting by Hasher.hash for unorderable elements). Added a test that hashes a frozenset in two subprocesses with different PYTHONHASHSEED and checks the hashes match, mirroring the existing set coverage.