Use newer Java features in places where it makes sense and more (#1009)

src/main/java/carpet/script/value/AbstractListValue.java

@@ -8,7 +8,7 @@ import java.util.List;
 
 public abstract class AbstractListValue extends Value implements Iterable<Value>
 {
-    public abstract Iterator<Value> iterator();
+    @Override public abstract Iterator<Value> iterator();
     public List<Value> unpack() { return Lists.newArrayList(iterator()); }
     public void fatality() { }
     public void append(Value v)