Various fixes for the lv4 backend:

 - expr such as "current x,y" should be written "current (x,y)"
 - abstract   struct  or   array   types  were   handled  as   extern
   types, which  prevent the struct  and array expanser to  work them
   out. In order to fix that, I have added an Abstract_type_eff variant
   to Eff.type_ which contains the concerte type.