Make some functions that should have been static static.

Fix a bunch of refleaks by fixing the definition of
MiddlingExtendsException.

Remove all the __new__ implementations apart from
BaseException_new.  Rewrite most code that needs it to cope with
NULL fields (such code could get excercised anyway, the
__new__-removal just makes it more likely).  This involved
editing the code for WindowsError, which I can't test.

This fixes all the refleaks in at least the start of a regrtest
-R :: run.