Test equals+hashCode contract  where overridden

Add tests for the equals+hashCode contract on all classes that have a custom definition (= not by identity and not genearted by Kotlins `data` keyword).

Affected classes:
https://github.com/prologdb/runtime/search?p=2&q=%22override+fun+equals%22&unscoped_q=%22override+fun+equals%22

Equals Verifier:
https://github.com/jqno/equalsverifier