For issue #58

          direct AST accessors

        * (Get_Core_Id): new function

        Continuation of issue #33

        For issue #33

          in new package Ocarina.Cmd_Line, rework all
          frontend/backends to avoid code duplication.

          For issue #33

          Also fix for issue #54

          Ocarina.Python_Cmd

          features.

          For issue #56