Macro handlers::handlers

macro_rules! handlers {
    (
        $system_name:ident {
            $($handler_name:ident {
                $($signal_name:ident ( $($arg_name:ident : $arg_type:ty),* ) => $slot_name:ident);*
            })*
        }
    ) => { ... };
    (
        DEFINE_HANDLERS $system_name:ident

        $($handler_name:ident {
            $($signal_name:ident ( $($arg_name:ident : $arg_type:ty),* ) => $slot_name:ident);*
        })*
    ) => { ... };
}