Implement add assignment for strings

examples/fetch.ds

@@ -1,4 +1,8 @@
 raw_data = (download "https://api.sampleapis.com/futurama/cast")
-dust_data = (from_json raw_data)
+cast_data = (from_json raw_data)
 
-(assert_equal "Billy West", dust_data.0.name)
+names = transform cast_member in cast_data {
+	cast_member.name
+}
+
+(assert_equal "Billy West", names.0)

src/abstract_tree/for.rs

@@ -32,12 +32,19 @@ 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();
+
+        let original_value = context.get_value(key)?;
 
         for value in list {
             context.set_value(key.clone(), value.clone())?;
 
-            self.item.run(source, &mut context)?;
+            self.item.run(source, context)?;
+        }
+
+        if let Some(original_value) = original_value {
+            context.set_value(key.clone(), original_value)?;
+        } else {
+            context.set_value(key.clone(), Value::Empty)?;
         }
 
         Ok(Value::Empty)

src/value/mod.rs

@@ -320,6 +320,7 @@ impl AddAssign for Value {
             (Value::Integer(left), Value::Integer(right)) => *left += right,
             (Value::Float(left), Value::Float(right)) => *left += right,
             (Value::Float(left), Value::Integer(right)) => *left += right as f64,
+            (Value::String(left), Value::String(right)) => *left += &right,
             _ => {}
         }
     }