so to summarize:

Proposed function signature:
```
PySendResult PyIter_Send(PyObject *obj, PyObject *arg, PyObject **result);
```

For generators/coroutines function will delegate to specialized implementation that does not raise StopIteration exception
For types that provide `tp_iternext` if arg is Py_None function call invoke `Py_TYPE(obj)->tp_iternext(obj)`
For all other cases function will try to call `send` method

Regarding of the case function will not raise StopIteration and will always return pair status/result.

Does it sound correct?