feat(lib): answer UserInfoRequest with our slots and queue

Opening a user's profile in SoulseekQt or Nicotine+ asks that peer
directly (peer code 15), and a peer that never answers shows an endless
spinner. The reply (code 16) carries no description or picture, the
configured upload slot count, how many uploads are waiting, and whether a
slot is free right now, which is what the reference clients send.

soulseek-rs-lib/src/message/peer/user_info.rs

@@ -0,0 +1,33 @@
+//! `UserInfoRequest` (peer code 15) and its reply (code 16): what a peer says
+//! about itself when another user opens its profile.
+
+use crate::message::{Message, MessageHandler};
+use crate::peer::PeerMessage;
+use std::sync::mpsc::Sender;
+
+pub struct UserInfoRequest;
+impl MessageHandler<PeerMessage> for UserInfoRequest {
+    fn get_code(&self) -> u32 {
+        15
+    }
+    fn handle(&self, _message: &mut Message, sender: Sender<PeerMessage>) {
+        let _ = sender.send(PeerMessage::UserInfoRequested);
+    }
+}
+
+/// Build a `UserInfoResponse` (peer code 16) with no description or picture.
+#[must_use]
+pub fn build_user_info(
+    upload_slots: u32,
+    queue_size: u32,
+    slots_free: bool,
+) -> Message {
+    Message::new()
+        .write_int32(16)
+        .write_string("")
+        .write_bool(false)
+        .write_int32(upload_slots)
+        .write_int32(queue_size)
+        .write_bool(slots_free)
+        .clone()
+}