bitv: Avoid undefined behavior in mask_for_bits

It was relying on the fact that x86 shifts are effectively %BITS, so
shifting a u32 by 32 preserves all bits.  But this is undefined behavior
in general.  Add an explicit %BITS to be safe.
Yeah, as long as `uint` wrapping is defined behavior, it could even be `(-bits) % u32::BITS`.  I expect the optimizer will already figure that out, but I can compare.