Merge --seller-addr and --seller-peer-id into --seller parameter

This simplifies the CLI's interface.

swap/src/libp2p_ext.rs

@@ -0,0 +1,15 @@
+use libp2p::multiaddr::Protocol;
+use libp2p::{Multiaddr, PeerId};
+
+pub trait MultiAddrExt {
+    fn extract_peer_id(&self) -> Option<PeerId>;
+}
+
+impl MultiAddrExt for Multiaddr {
+    fn extract_peer_id(&self) -> Option<PeerId> {
+        match self.iter().last()? {
+            Protocol::P2p(multihash) => PeerId::from_multihash(multihash).ok(),
+            _ => None,
+        }
+    }
+}