gh-149807 : frozendict_hash : hash (key, value) pairs rather than keys and values separately

[lambda-abstraction]

i used the simple old boost hash_combine rather than directly reusing the tuple xxhash because i think its overkill for the case of combining 2 hashes in a structure where the first elements are different
im sure there is an equally cheap and simple but a better distributed combination function for 2 hashes, but im not sure what it is, so thats open for modification

• Issue: frozendict_hash doesnt match the PEP and might have too many collisions #149807

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[lambda-abstraction]

I think making combine_hashes a specialization unroll of tuple xxhash for 2 elements could be good, to avoid creating a tuple and the branching of the loop while using an already trusted algorithm