Merge: model: register the implicitly injected Bool class in its hierarchy.

The model can inject a primitive a Bool class when required.
It was a hack which was initially introduced by the interpreter that need to manipulate boolean values even if they where not present in the original program.
However, the class was not fully initialized. So fix that.

Maybe a future PR will try to remove this hack (it is not the model's job to inject classes)

Pull-Request: #989
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc src/interpreter/naive_interpreter.nit
Simple merge