TODO list for Sidef

General:
    * improve the OO system
    * implement sidef in Sidef (too optimistic?)

Classes:
    * implement roles (from Perl 6) and add the `does` trait for classes
      (http://doc.perl6.org/language/objects#Roles)

Blocks:
    * find a better way to implement the `next` and `last` statements

Method/functions:
    * implement the `is export` trait for functions and classes (maybe)
