Quality control, meet exceptions.c.

Fix a number of problems with the need for speed code:

One is doing this sort of thing:

    Py_DECREF(self->field);
    self->field = newval;
    Py_INCREF(self->field);

without being very sure that self->field doesn't start with a
value that has a __del__, because that almost certainly can lead
to segfaults.

As self->args is constrained to be an exact tuple we may as well
exploit this fact consistently.  This leads to quite a lot of
simplification (and, hey, probably better performance).

Add some error checking in places lacking it.

Fix some rather strange indentation in the Unicode code.

Delete some trailing whitespace.

More to come, I haven't fixed all the reference leaks yet...