python: native functions calling to python functions are not displayed

Native functions can be functions of the Python runtime, or Cythonized functions.

py-spy does not collect native frames at all right now (it supports collection but we don't enable this mode, partially because I'm under impression it's very inaccurate under `--nonblocking` mode).
PyPerf does collect native frames, but we stop at the first Python function (i.e the first `PyEval_EvalFrame*`).

If we have a stack like:
`python; pyfunc1; pyfunc2; cythonfunc3; cythonfunc4; pyfunc5 ...`
then both py-spy and PyPerf will collect it as `python; pyfunc1; pyfunc2; pyfunc5 ...`.

I don't have a good idea for it now, perhaps PyPerf can collect deeper native frames and correlate frames also with frames higher in the stack than the first Python frames?