(struct bitset_vtable.size, struct bitset_vtable.count,

(struct bitset_vtable.size, struct bitset_vtable.count,
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.