De-abstract Savable by making it a protocol

[unkcpz]

The outcome of this refactoring is that the Savable become a protocol which provide the contract for how things needed to be saved should implemented.

@runtime_checkable
class Savable(Protocol):
    @classmethod
    def recreate_from(cls, saved_state: SAVED_STATE_TYPE, load_context: LoadSaveContext | None = None) -> 'Savable':
        """
        Recreate a :class:`Savable` from a saved state using an optional load context.

        :param saved_state: The saved state
        :param load_context: An optional load context

        :return: The recreated instance

        """
        ...

    def save(self, save_context: LoadSaveContext | None = None) -> SAVED_STATE_TYPE: ...

Compare to the original implementation where save calls save_instance_state calls each other recursively until all savable things is saved, the new implementation force the save method independent and have symmetry behavior of recreate_from.