WIP: udp: add ipv6 support

Returning IPv6 peers doesn't really work with UDP. It is not supported by
https://libtorrent.org/udp_tracker_protocol.html. There is a suggestion in
https://web.archive.org/web/20170503181830/http://opentracker.blog.h3q.com/2007/12/28/the-ipv6-situation/
of using action number 4 and returning IPv6 octets just like for IPv4
addresses. Clients seem not to support it very well, but due to a lack of
alternative solutions, it is implemented here

aquatic_udp/src/lib/network.rs

@@ -1,6 +1,6 @@
 use std::sync::{Arc, atomic::{AtomicUsize, Ordering}};
 use std::io::{Cursor, ErrorKind};
-use std::net::SocketAddr;
+use std::net::{SocketAddr, IpAddr};
 use std::time::Duration;
 use std::vec::Drain;
 
@@ -215,7 +215,9 @@ fn send_responses(
     for (response, src) in response_iterator {
         cursor.set_position(0);
 
-        response_to_bytes(&mut cursor, response, IpVersion::IPv4).unwrap();
+        let ip_version = ip_version_from_ip(src.ip());
+
+        response_to_bytes(&mut cursor, response, ip_version).unwrap();
 
         let amt = cursor.position() as usize;
 
@@ -240,4 +242,18 @@ fn send_responses(
         state.statistics.bytes_sent
             .fetch_add(bytes_sent, Ordering::SeqCst);
     }
+}
+
+
+fn ip_version_from_ip(ip: IpAddr) -> IpVersion {
+    match ip {
+        IpAddr::V4(_) => IpVersion::IPv4,
+        IpAddr::V6(ip) => {
+            if let [0, 0, 0, 0, 0, 0xffff, ..] = ip.segments(){
+                IpVersion::IPv4
+            } else {
+                IpVersion::IPv6
+            }
+        }
+    }
 }