Macro hello_egui_utils::fnify

macro_rules! fnify {
    (
        $(#[$docs:meta])*
        $name:ident,
        body: $body:block,
        parameters: ($($arg:ident: $typ:ty,)*),
        async_body: $async_body:block,
        async_parameters: ($(;$async_mutt:ident)? $($async_arg:ident: $async_typ:ty,)*),
        call_args: ($($call_args:ident,)*),
        generics: ($($gen:tt)*),
        async_generics: ($($async_gen:tt)*),
        return_type: ($($return_type:tt)*),
        call_prefix: ($($call_prefix:tt)*),
        callback_body_self: ($($callback_body_self:tt)*),
    ) => { ... };
}

This macro generates the async fn and the callback fn