Require that the hash node must be the first element of the structure to be

Require that the hash node must be the first element of the structure to be
managed in a hash table. This gives smaller code and a ~25% size reduction of
the HashNode structure which might become an advantage if many elements are
hashed.

git-svn-id: svn://svn.cc65.org/cc65/trunk@5158 b7a2c559-68d2-44c3-8de9-860c34a00d81