Oop-ext

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* ``Callbacks`` can be used as context manager, which provides a ``Register(callback, function)``,
which automatically unregisters all functions when the context manager ends.

* ``Callback.Register(function)`` now returns an object with a ``Unregister()`` method, which
can be used to undo the register call.

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* Change back the default value of ``requires_declaration`` to ``True`` and fix an error (22) where the cache wasn't properly cleared.

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* Fixes an issue (20) where mocked `classmethods` weren't considered a valid method during internal checks.

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* Add optional argument ``requires_declaration`` so users can decide whether or not ``ImplementsInterface`` declarations are necessary.

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* Implementations no longer need to explicitly declare that they declare an interface with ``ImplementsInterface``: the check is done implicitly (and cached) by `AssertImplements` and equivalent functions.

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* Interface and implementation methods can no longer contain mutable defaults, as this is considered
a bad practice in general.

* ``Null`` instances are now hashable.