sbr/vector.c: Only allocate bvector storage for set bits.

sbr/vector.c: Only allocate bvector storage for set bits.

Now that a pointer to all the bit storage doesn't escape vector.c,
there's no need to allocate storage, that's initialised to 0, just to
then clear a bit in it.  Only extend the bits's storage for set bits.
Remove the unused initial size parameter from bvector_create() as it
simplifies the implementation.