struct bitset_vtable.list, struct bitset_vtable.list_reverse,
bitset_count_, bitset_next, bitset_prev, bitset_first, bitset_last,
bitset_count_):
Use bitset_bindex, not int or unsigned int or size_t, to count bits.
(BITSET_BINDEX_MAX, BITSET_WINDEX_MAX, BITSET_SIZE_MAX): New macros.
(BITSET_INDEX_MAX): Remove. It wasn't right, since it assumed that
sizeof (bitset_word) == sizeof (bitset_windex). All uses changed
to BITSET_WINDEX_MAX.
size_t; the old version tried to do this but casted improperly.
(bitset_bindex, bitset_windex): Now size_t, not unsigned long.
(bitset_test): Now returns int, not unsigned long.
