set_bits_t is now 64 bits for x86_64 machines (in linux, anyway). This gave
qfvis a huge speed boost: from ~815s to ~720s.
Also, expose some of the set internals so custom set operators can be
created.
Getting everything right with an enum proved to be too difficult if not
impossible. Also use better tests for equivalence and intersection.
Many more tests have been added. All pass :)
