an object as a non-negative integer.  Code trying to pass addresses to
an %x format uses positive_id(), and this avoids a Python FutureWarning
about applying %x to negative ints.  The primary difference between this
and the last stab is that positive_id() should work OK on 64-bit boxes
too.  What we really want here is C's %p format code, but in Python we
can't even reliably know the width of native addresses.