Scaffold the federated services daemon

Protocol-agnostic core: a Protocol trait maps raw server-to-server lines to a
normalized event/action model, so the engine never touches a raw line and a new
ircd is one new module. Adds event-log state (state is a fold over the log) and
a service framework. First protocol impl links to an InspIRCd (insp4) uplink —
completes the CAPAB/SERVER handshake, bursts, and introduces NickServ.

src/engine/service.rs

@@ -0,0 +1,29 @@
+use crate::proto::NetAction;
+
+// A pseudo-client (NickServ, ChanServ, OperServ, ...). Introduced at burst,
+// receives the commands users message it, and pushes actions onto the ctx.
+pub trait Service: Send {
+    fn nick(&self) -> &str;
+    fn uid(&self) -> &str;
+    fn host(&self) -> &str {
+        "services.local"
+    }
+    fn gecos(&self) -> &str;
+    fn on_command(&mut self, from: &str, args: &[&str], ctx: &mut ServiceCtx);
+}
+
+// What a service can do in response to a command, collected for the link to flush.
+#[derive(Default)]
+pub struct ServiceCtx {
+    pub actions: Vec<NetAction>,
+}
+
+impl ServiceCtx {
+    pub fn notice(&mut self, from: &str, to: &str, text: impl Into<String>) {
+        self.actions.push(NetAction::Notice {
+            from: from.to_string(),
+            to: to.to_string(),
+            text: text.into(),
+        });
+    }
+}