gcc has a special case to devolve to memset if both the pointed-at data
and the value to fill are considered a byte. All uses of
DXX_POISON_MEMORY pass a value that fits in a byte, so change the
signature to be a byte, to encourage gcc to activate the special case.
This removes the need to walk all objects when creating a new one, since
each object can have a private generation counter, unaware of other
objects. For compatibility with demos, mix in the object's index when
writing the signature value.
