Futher improvements to frozenset hashing (based on Yitz Gale's battery of

Futher improvements to frozenset hashing (based on Yitz Gale's battery of
tests which nicely highly highlight weaknesses).

* Initial value is now a large prime.
* Pre-multiply by the set length to add one more basis of differentiation.
* Work a bit harder inside the loop to scatter bits from sources that
  may have closely spaced hash values.

All of this is necessary to make up for keep the hash function commutative.
Fortunately, the hash value is cached so the call to frozenset_hash() will
only occur once per set.