Improve for loop implementation

examples/for_loop.ds

@@ -1,3 +1,6 @@
+i = 0
 for i in [1 2 3] {
+	i += 1
 	(output i)
 }
+(output i)

src/abstract_tree/for.rs

@@ -31,15 +31,14 @@ impl AbstractTree for For {
         let value = self.expression.run(source, context)?;
         let list = value.as_list()?;
         let key = self.identifier.inner();
+        let mut context = context.clone();
 
         for value in list {
             context.set_value(key.clone(), value.clone())?;
 
-            self.item.run(source, context)?;
+            self.item.run(source, &mut context)?;
         }
 
-        context.set_value(key.clone(), Value::Empty)?;
-
         Ok(Value::Empty)
     }
 }