ext/standard: Improve checking of allowed_classes option (#15267)

* ext/standard: Add some unserializing tests

* ext/standard: Add proper type checking for values of the allowed_cla

ext/standard: Improve checking of allowed_classes option (#15267)

* ext/standard: Add some unserializing tests

* ext/standard: Add proper type checking for values of the allowed_classes option array

* ext/standard: Check that class names are somewhat sensible for the allowed_classes option array

* Indicate type of value

* Add test for Stringable objects

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