completely changed the VData/VDataEnum system: Multiple VData can share one VDataEnum via an Arc<Mutex<VDataEnum>>. If one wants to modify the data, it copies it (Copy on Write). This copying is skipped for shared mutable data (references). This was necessary for the get_ref() function. Expect bugs!

mers/tests/test_in_mers.rs

@@ -15,7 +15,7 @@ fn run_all() {
                 let mut file = File::new(fs::read_to_string(file.path()).unwrap(), file.path());
                 // has to return true, otherwise the test will fail
                 assert!(matches!(
-                    parse::parse(&mut file).unwrap().run(vec![]).data,
+                    parse::parse(&mut file).unwrap().run(vec![]).data().0,
                     VDataEnum::Bool(true)
                 ));
             }