Implement the Bridge class to translate between the two protocols

Add all useful classes as well: Jid, Iid, IrcChannel, IrcUser etc to
properly keep the informations about what we receive from the IRC server.

Only handle the MUC join stanza, and send the list of users in the IRC
channel to the XMPP user, and the IRC channel’s topic, for now.

src/irc/irc_channel.hpp

@@ -0,0 +1,34 @@
+#ifndef IRC_CHANNEL_INCLUDED
+# define IRC_CHANNEL_INCLUDED
+
+#include <irc/irc_user.hpp>
+#include <memory>
+#include <string>
+#include <vector>
+
+/**
+ * Keep the state of a joined channel (the list of occupants with their
+ * informations (mode, etc), the modes, etc)
+ */
+class IrcChannel
+{
+public:
+  explicit IrcChannel();
+
+  bool joined;
+  std::string topic;
+  void set_self(const std::string& name);
+  IrcUser* get_self() const;
+  IrcUser* add_user(const std::string& name);
+
+private:
+  std::unique_ptr<IrcUser> self;
+  std::vector<std::unique_ptr<IrcUser>> users;
+
+  IrcChannel(const IrcChannel&) = delete;
+  IrcChannel(IrcChannel&&) = delete;
+  IrcChannel& operator=(const IrcChannel&) = delete;
+  IrcChannel& operator=(IrcChannel&&) = delete;
+};
+
+#endif // IRC_CHANNEL_INCLUDED