Use an intermediate function to build the handlers

Such a function may later be generated using a procedural macro.

examples/dummy.rs

@@ -1,14 +1,21 @@
 use log::debug;
 use env_logger::{Builder, Env};
-use opensmtpd::{Entry, EventHandler, MatchEvent, SmtpIn};
+use opensmtpd::{handlers, Entry, EventHandler, MatchEvent, SmtpIn};
 
 fn on_event(entry: &Entry) -> bool {
     debug!("Event received: {:?}", entry);
     true
 }
 
+// This function should be replaced by a procedural macro on
+// the `on_event` function.
+fn on_event_builder() -> EventHandler {
+    EventHandler::new(MatchEvent::All, on_event)
+}
+
 fn main() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
-    let h = vec![EventHandler::new(MatchEvent::All, on_event)];
-    SmtpIn::new().event_handlers(h).run();
+    SmtpIn::new()
+        .event_handlers(handlers!(on_event_builder))
+        .run();
 }