Merge: No duplicated mclassdefs

This series avoid the creation of multiple mclassdefs for a same class in a same module.

It appended for implicit classes: Object for top-level methods and Sys for top-level main.

The series was more complex than expected because it breaks the symmetry between AClassdef and MClassDef. However, thanks to the separation of the AST and the model, it was sill feasible.

Pull-Request: #503
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>