Rewrite the project

The previous project architecture was far too complicated and hard to
maintain. The new one is much more simple. Although procedural macros
are cools, they are a no-go on Rust-OpenSMTPD.

Reports and filter are implemented (except data-line) but untested.

examples/counter.rs

@@ -0,0 +1,45 @@
+use log;
+use opensmtpd::{register, run_filter, Address, Filter, ReportEntry};
+use simplelog::{Config, LevelFilter, TermLogger, TerminalMode};
+
+#[derive(Default)]
+struct MyCounter {
+    nb_connected: u64,
+    nb_total: u64,
+}
+
+impl Filter for MyCounter {
+    register!(has_report_link_connect);
+    fn on_report_link_connect(
+        &mut self,
+        _entry: &ReportEntry,
+        _rdns: &str,
+        _fcrdns: &str,
+        _src: &Address,
+        _dest: &Address,
+    ) {
+        self.nb_connected += 1;
+        self.nb_total += 1;
+        log::info!(
+            "New client (connected: {}, total: {})",
+            self.nb_connected,
+            self.nb_total
+        );
+    }
+
+    register!(has_report_link_disconnect);
+    fn on_report_link_disconnect(&mut self, _entry: &ReportEntry) {
+        self.nb_connected -= 1;
+        log::info!(
+            "Client left (connected: {}, total: {})",
+            self.nb_connected,
+            self.nb_total
+        );
+    }
+}
+
+fn main() {
+    TermLogger::init(LevelFilter::Debug, Config::default(), TerminalMode::Mixed).unwrap();
+    let mut my_counter: MyCounter = Default::default();
+    run_filter(&mut my_counter);
+}

examples/samples/empty.log

@@ -0,0 +1,4 @@
+config|smtpd-version|6.6.1
+config|smtp-session-timeout|300
+config|subsystem|smtp-in
+config|ready

examples/samples/single_session.log

@@ -0,0 +1,6 @@
+config|smtpd-version|6.6.1
+config|smtp-session-timeout|300
+config|subsystem|smtp-in
+config|ready
+report|0.5|1576146008.006099|smtp-in|link-connect|7641df9771b4ed00|mail.openbsd.org|pass|199.185.178.25:33174|45.77.67.80:25
+report|0.5|1576147242.200225|smtp-in|link-disconnect|7641dfb3798eb5bf