C++: use struct for our private implementations rather than class

[agarny]

Component::ComponentImpl, for instance, is declared using the class keyword. Yet, we only use public access. So, we could use struct instead of class (and thus not have to specify public access), as we do for Generator::GeneratorImpl for instance.

So, here, we want to replace all of our occurrences of class with struct when it comes to the private implementation of our various classes.