bitv: Fix all() for nbits that are multiples of u32::BITS

bitv: Fix all() for nbits that are multiples of u32::BITS

The old logic would be ok with *either* 0 or all 1s in the last word,
because it didn't compute a proper mask for the case where nbits is an
exact multiple of u32::BITS.

Add mask_for_bits() to compute this properly, and use it in all().  Add
all/none assertions to most of the tests.  Note in particular, the all-zero
bitv in test_32_elements() was incorrectly all()==true before this patch.