StartObject initializes bookkeeping for writing a new object.

def StartObject(self, numfields):
        """StartObject initializes bookkeeping for writing a new object."""

        self.assertNotNested()

        # use 32-bit offsets so that arithmetic doesn't overflow.
        self.current_vtable = [0 for _ in range_func(numfields)]
        self.objectEnd = self.Offset()
        self.nested = True