'WorkoutStoreProtocol' should not be only for classes

[indisoluble]

'WorkoutStoreProtocol' defines the interface for a store, so it will be more likely implemented by a class given that a struct would be copied on each assignment instead of being shared by reference. However, the reason why I forced this protocol to be conformed only by classes is because I pass a 'WorkoutStoreProtocol' instance to 'ListWorkoutsModuleConfigurator' where I set the property 'delegate' to a value. Unless I specify this protocol is only for classes, Swift assumes it could be a struct passed as a parameter to a function, in which case (and unless I also specify it is a inout param) I am not allowed to modify it.