Rather than:

  f = create_frame_or_gen(...);
  if (<is generator>) return f;
  retval = _PyEval_EvalFrame(tstate, f, 0);
  _PyObject_GC_TRACK(f);
  return retval;

I was thinking:

  f = create_frame(...);
  if (<is generator>) return make_gen(f);
  retval = _PyEval_EvalFrame(tstate, f, 0);
  _PyObject_GC_TRACK(f);
  return retval;

The complicated part is create_frame(...), so I want to clean that up first.