When syn/clone-impls isn't enabled, this type is not Copy.

    warning: this argument (4 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
      --> src/lifetime.rs:22:49
       |
    22 |     fn visit_opt_lifetime(&mut self, reference: &Token![&], lifetime: &mut Option<Lifetime>) {
       |                                                 ^^^^^^^^^^ help: consider passing by value instead: `$crate::token::And`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref
       = note: `-W clippy::trivially-copy-pass-by-ref` implied by `-W clippy::pedantic`
       = help: to override `-W clippy::pedantic` add `#[allow(clippy::trivially_copy_pass_by_ref)]`