Constant evulator for next_power_of_two in proc. macros.

const_eval_for_next_power_of_two.rs

fn const_next_power_of_two(ty: TokenStream2, value: TokenStream2) -> TokenStream2 {
    quote! {{
        if (#value) <= 1 {
            0
        } else {
            let p = (#value) - 1;
            let z = p.leading_zeros();
            <#ty>::MAX as usize >> z
        }
    }}
}