Merge: Serialization phase skips abstract classes and add depth option to nitserial

Merge: Serialization phase skips abstract classes and add depth option to nitserial

This is a minor fix and new feature for the serialization support.

* Prevents bug where the serialization phase create constructors for abstract classes.
* Nitserial generates the generics support code for different depths: single module, group or project.

Note that the serialization does not support fully the new constructors. But it can still be used on small classes that do not use any "fancy" constructor features.

Pull-Request: #1193
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>