Move the private ``_PyFrameEvalFunction`` type, and private
``_PyInterpreterState_GetEvalFrameFunc()`` and
``_PyInterpreterState_SetEvalFrameFunc()`` functions to the internal C API. The
``_PyFrameEvalFunction`` callback function type now uses the
``_PyInterpreterFrame`` type which is part of the internal C API. Patch by
Victor Stinner.
