Fix this error in the new Rust Nightly:
```rust
Compiling script v0.0.1 (file:///home/simon/servo2/components/script)
error[E0599]: no method named `trace` found for type `&fn(&dom::node::Node) -> u16` in the current scope
--> /home/simon/servo2/components/script/dom/treewalker.rs:464:10
|
464 | #[derive(JSTraceable)]
| ^^^^^^^^^^^
|
= note: JSTraceable is a function, perhaps you wish to call it
= help: items from traits can only be used if the trait is implemented and in scope
= note: the following trait defines an item `trace`, perhaps you need to implement it:
candidate #1: `dom::bindings::trace::JSTraceable`
error: aborting due to previous error
error: Could not compile `script`.
```
~I’ve tried to reproduce it in a minimal test case, but `impl<A, B> SomeTrait for fn(A) -> B` did not apply for `fn(&u32) -> u32` even on Rust 1.19.0 stable. So I don’t know what changed.~ This is very likely https://github.com/servo/servo/pull/18327.
Source-Repo: https://github.com/servo/servo
Source-Revision: fd833d9f18cd9196502b9768ab0ac8c1498b2f68
