Fix palette! macro

[rgiot]

It fails when used in one project:

56 |           let palette = palette![
   |  _______________________^
57 | |                 "black".to_owned(),
58 | |                 "blue".to_owned(), "bright blue".to_owned(), "sky blue".to_owned(),
59 | |                 "red".to_owned(), "bright red".to_owned(), "purple".to_owned(),
60 | |                 "green".to_owned(), "bright green".to_owned(), "pastel green".to_owned()
61 | |         ];
   | |_________^ the trait `num_integer::Integer` is not implemented for `&cpc::Pen`