For some algorithms, such as bloom filters, it's possible to reuse a
hash function (rather than having different hashing functions) if the
seed is different each time the hash function is used.
Modify AK::string_hash() to take a seed parameter, which defaults to 0
(the value the hash value was originally initialized to).
Example failure:
IDAllocator.h only pulls in AK/Hashtable.h, so any compilation unit that
includes AK/IDAllocator.h without including AK/Traits.h before it used
to be doomed to fail with the cryptic error message "In instantiation of
'AK::HashTable<T, TraitsForT, IsOrdered>::Iterator AK::HashTable<T,
TraitsForT, IsOrdered>::find(const T&) [with T = int; TraitsForT =
AK::Traits: incomplete type 'AK::Traits<int>' used in nested name
specifier".
