Fixes for nitg-s and standard lib

In nitg-s:
    * Fix the non-working call to super from primitive types
    * Fix support for abstract methods in extern classes 
Standard lib:
    * Fix Char.to_i support for letter (hex mainly)
    * TODO update doc of Char::to_i or add a specialized method for this usage

Les 3 commits passent séparément les tests, voir #13, #14, #15 à http://gresil.org/jenkins/job/CI%20Alexis/changes