Allow the use of custom context

The main goal of events/reports is to update a context object, which
will be used in filters to generate a response. It is now possible to
use any object implementing both Clone and Default as a context object.
It is also possible to define no context at all.

opensmtpd/examples/dummy.rs

@@ -1,14 +1,14 @@
 use env_logger::{Builder, Env};
 use log::{debug, info};
-use opensmtpd::{event, handlers, Entry, NoContext, SmtpIn};
+use opensmtpd::{event, handlers, Entry, SmtpIn};
 
 #[event(Any)]
-fn on_event(_context: &mut NoContext, entry: &Entry) {
+fn on_event(entry: &Entry) {
     debug!("Event received: {:?}", entry);
 }
 
 #[event(LinkConnect)]
-fn on_connect(_context: &mut NoContext, entry: &Entry) {
+fn on_connect(entry: &Entry) {
     info!("New client on session {:x}.", entry.session_id);
 }
 

opensmtpd/examples/session_event_counter.rs

@@ -0,0 +1,19 @@
+use env_logger::{Builder, Env};
+use log::info;
+use opensmtpd::{event, handlers, Entry, SmtpIn};
+
+#[derive(Clone, Default)]
+struct MyContext {
+    nb: usize,
+}
+
+#[event(Any)]
+fn on_event(ctx: &mut MyContext, entry: &Entry) {
+    ctx.nb += 1;
+    info!("Event received: {}, {}", entry.session_id, ctx.nb);
+}
+
+fn main() {
+    Builder::from_env(Env::default().default_filter_or("debug")).init();
+    SmtpIn::new().event_handlers(handlers!(on_event)).run();
+}