stcp: ed25519 pubkey exchange in handshake, fix instance destroy segfault, fix chat_tcp_test

- u_async.c: process immediate_queue BEFORE epoll_wait in uasync_poll
- stcp_link.c: defensive NULL etcp_conn/etcp_link in stcp_link_close
- utun_instance.c: move stcp_server_list_destroy_all to Phase J (before ETCP),
  add deferred drain in both Phase J and L
- stcp.h/c: STCP_HS_ENC_CLIENT 6→38, STCP_HS_ENC_SERVER 7→39,
  exchange ed25519_pubkey in both handshake directions
- etcp_connections.c: copy peer_ed25519 from STCP to ETCP_CONN in
  etcp_link_enter_ready_tcp; guard link->etcp in tcp_link_close_cb
- chat_sync.c: fix lk->conn NULL dereference for TCP links
- test_stcp.c: update stcp_server_create/stcp_client_connect callsites
- tools/chat_tcp_test: change transport udp→tcp

tools/chatgui/zxing-cpp/core/src/TextEncoder.h

@@ -0,0 +1,31 @@
+/*
+* Copyright 2016 Nu-book Inc.
+*/
+// SPDX-License-Identifier: Apache-2.0
+
+#pragma once
+
+#include "CharacterSet.h"
+
+#include <string>
+
+namespace ZXing {
+
+class TextEncoder
+{
+	static void GetBytes(const std::string& str, CharacterSet charset, std::string& bytes);
+	static void GetBytes(const std::wstring& str, CharacterSet charset, std::string& bytes);
+public:
+	static std::string FromUnicode(const std::string& str, CharacterSet charset) {
+		std::string r;
+		GetBytes(str, charset, r);
+		return r;
+	}
+	static std::string FromUnicode(const std::wstring& str, CharacterSet charset) {
+		std::string r;
+		GetBytes(str, charset, r);
+		return r;
+	}
+};
+
+} // ZXing