Raise OverflowError when appropriate on long->float conversion. Most of

Raise OverflowError when appropriate on long->float conversion.  Most of
the fiddling is simply due to that no caller of PyLong_AsDouble ever
checked for failure (so that's fixing old bugs).  PyLong_AsDouble is much
faster for big inputs now too, but that's more of a happy consequence
than a design goal.