Make SimpleAssignmentsStorage::$assignments protected

[arogachev]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[what-the-diff]

PR Summary

• Enhanced Type Checking in Manager.php
  The code now enforces more rigorous type checking to avoid potential errors. Specifically, if the 'parent' object is a 'Permission' type and the 'child' object is a 'Role' type, certain exceptions would occur previously. With the new change, the system conducts an 'instance check'. Only if the 'parent' is a 'Permission' instance and the 'child' a 'Role' instance, the system proceeds; otherwise, a runtime error is flagged to alert the user of a problem.

• Visibility Modification in SimpleAssignmentsStorage.php
  In an effort to enhance the flexibility of our code, we have changed the visibility status of the 'assignments' attribute. It was previously marked as 'private', restricting its accessibility outside the class. We have now made it 'protected', which allows it to be accessed not only within the class, but also in any derived classes. This change promotes code modularity and reusability while maintaining a reasonable level of data protection and encapsulation.