Add __slots__ to structs about ctypeslib

[mara004]

Note that __slots__ must be defined in the actual class body in order to be effective, that's very important.
The above post-definition assignment does nothing except set a casual attribute, i.e. won't enable the special slots feature.

Minimal repro:

>>> from ctypes import *
>>>
>>> class TestStruct (Structure): pass
... 
>>> TestStruct.__slots__ = ["a", "b"]
>>> s = TestStruct()
>>> s.c = 1
>>> 
>>> class TestStruct (Structure):
...     __slots__ = ["a", "b"]
... 
>>> s = TestStruct()
>>> s.c = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'TestStruct' object has no attribute 'c'