Action/Layout: Explicit the Debug trait to be able to use it

[borisfaure]

Even though some types derive Debug, T/K need to implement it.

error[E0277]: `T` doesn't implement `Debug`
   --> src/layout.rs:89:25
    |
89  |     layers: &'static [[[Action<T, K>; C]; R]; L],
    |                         ^^^^^^^^^^^^ `T` cannot be formatted using `{:?}` because it doesn't implement `Debug`
    |
note: required by a bound in `Action`