Macro step_integer_impls

macro_rules! step_integer_impls {
    {
        narrower than or same width as usize:
            $( [ $u_narrower:ident $i_narrower:ident ] ),+;
        wider than usize:
            $( [ $u_wider:ident $i_wider:ident ] ),+;
    } => { ... };
}