- Expose using slot based generated code to load entry points
- Add first tests
- TODO:
    - Fix possible cache poisoning when loading slot vs no slot structs
    - Add tests for above
    - Add pickling tests for slot based structs

- Replace class with a newer one with slot fields
- Implement subclass and instance checks

During the first creation of the loaded class with slots, a new class is
inserted into the inheritance chain to ensure that the slot fields are defined.

This is done because of the need to create an empty struct during the
parsing phase and fill it in later - slots require the fields to be known before hand.

All checks on the new replacement class will have it looking like the
original except an equality comparison between the replaced class and
its replacement.

>>> import thriftpy
>>> ab = thriftpy.load('addressbook.thrift', use_slots=True)
>>> ab_inst = ab.AddressBook()
>>> ab_inst.__class__ == ab.AddressBook # will return False
>>> # all other checks should work as expected
>>> isinstance(ab_inst, ab.AddressBook) # will return True
>>> issubclass(ab_inst.__class__, ab.AddressBook) # will return True

In order to get pickling to work as expected, a new extension type is
registered with copyreg (copy_reg for py2x) to avoid pickling errors.