check_missing_items.py: Don't overwrite ty in loop

Because python doesn't have lexical scope, loop variables
persist after the loop is exited, set to the value of the last
itteration

```
>>> i = 0
>>> for i in range(10): pass
...
>>> i
9
```

This causes the `ty` variable to be changed, causing unexpected crashes on
```
pub type RefFn<'a> = &'a dyn for<'b> Fn(&'a i32) -> i32;
```

src/etc/check_missing_items.py

@@ -88,8 +88,8 @@ def check_type(ty):
                         for bound in binding["binding"]["constraint"]:
                             check_generic_bound(bound)
             elif "parenthesized" in args:
-                for ty in args["parenthesized"]["inputs"]:
-                    check_type(ty)
+                for input_ty in args["parenthesized"]["inputs"]:
+                    check_type(input_ty)
                 if args["parenthesized"]["output"]:
                     check_type(args["parenthesized"]["output"])
         if not valid_id(ty["inner"]["id"]):