Assume unused/unreachable bits are zero.

The more I study the code, the less I understand the comment,

    ; TODO: check last word

I'm not sure why the `for/and` loop treated "the last word" as the first
byte. More basically I'm not sure why the byte with potentially unused
bits needs to be handled differently at all. As best I can tell, the
implementation has the invariant that the unused bits of the last word
will be zero.

This commit uses that assumption. The `for/and` loop simply compares all
the bytes.

Also: Delete comment about 30 or 62 bits per word. AFAICT that seems to
be N/A because the bit vector is simply stored in 8-bit bytes.