Introduce new private API function _PyLong_AsScaledDouble. Not used yet,

Introduce new private API function _PyLong_AsScaledDouble.  Not used yet,
but will be the foundation for Good Things:
+ Speed PyLong_AsDouble.
+ Give PyLong_AsDouble the ability to detect overflow.
+ Make true division of long/long nearly as accurate as possible (no
  spurious infinities or NaNs).
+ Return non-insane results from math.log and math.log10 when passing a
  long that can't be approximated by a double better than HUGE_VAL.