Tweak fasdumper to null-terminate bytevectors too.

Tweak fasdumper to null-terminate bytevectors too.

find_primitive relies on symbol names (which were formerly `strings',
i.e. `vector-8b' or TC_CHARACTER_STRING, and are now `bytevectors')
to be null-terminated.

Rather than try to figure out which ones are primitive names and
which ones are not, we'll just null-terminate all of them.

We can undo this if we change find_primitive.  (We need to change the
compile-time tool findprim.c anyway because its strcmp_ci misuses
islower/toupper.)