The original hash algorithm just XOR'd all the hashes of the elements of
the set, which is problematic. The CPython algorithm is required to pass
the tests.
- Replace `PyFrozenSet::hash` with CPython's algorithm
- Remove unused `hash_iter_unorded` functions
- Add `frozenset` benchmark
- Enable tests
- Lower performance expectations on effectiveness test
- Adjust `slot::hash_wrapper` so that it doesn't rehash the computed
hash value in the process of converting PyInt to PyHash.