Code quality: destructors of static global objects will no longer call external dependencies. Manual cleaning possible through backend::release()

This should solve errors when libOpenCL's ICD was unloaded prior to libisaac.

include/isaac/symbolic/expression.h

@@ -238,7 +238,7 @@ struct execution_options_type
   {
     if(queue_)
         return *queue_;
-    return driver::backend::queues(context)[queue_id_];
+    return driver::backend::queue(context, queue_id_);
   }
 
   std::list<driver::Event>* events;