Cache hash value for FrozenSets

Adds a hash field to the `PyFrozenSet` data type in order to avoid
recomputing the hash of an immutable object.

Lib/test/test_set.py

@@ -728,15 +728,7 @@ class TestFrozenSet(TestJointOps, unittest.TestCase):
             for i in range(len(s)+1):
                 yield from map(frozenset, itertools.combinations(s, i))
 
-        # TODO: RUSTPYTHON
-        # The original test has:
-        # for n in range(18):
-        # Due to general performance overhead, hashing a frozenset takes
-        # about 50 times longer than in CPython. This test amplifies that
-        # exponentially, so the best we can do here reasonably is 13.
-        # Even if the internal hash function did nothing, it would still be
-        # about 40 times slower than CPython.
-        for n in range(13):
+        for n in range(18):
             t = 2 ** n
             mask = t - 1
             for nums in (range, zf_range):