contact: implement to_string

Make it easy to get a string for a sequence of contacts.

lib/message/mu-contact.cc

@@ -19,6 +19,7 @@
 
 #include "mu-contact.hh"
 #include "mu-message.hh"
+#include "utils/mu-utils.hh"
 
 #include <gmime/gmime.h>
 #include <glib.h>
@@ -85,6 +86,21 @@ Mu::make_contacts(const std::string& addrs,
 }
 
 
+std::string
+Mu::to_string(const Mu::Contacts& contacts)
+{
+	std::string res;
+
+	seq_for_each(contacts, [&](auto&& contact) {
+		if (res.empty())
+			res = contact.display_name();
+		else
+			res += ", " + contact.display_name();
+	});
+
+	return res;
+}
+
 size_t
 Mu::lowercase_hash(const std::string& s)
 {

lib/message/mu-contact.hh

@@ -164,6 +164,17 @@ make_contacts(/*const*/ struct _InternetAddressList* addr_lst,
 Contacts
 make_contacts(const std::string& addrs,
 	      Field::Id field_id, int64_t message_date);
+
+
+/**
+ * Get contacts as a comma-separated list.
+ *
+ * @param contacts contacs
+ *
+ * @return string with contacts.
+ */
+std::string to_string(const Contacts& contacts);
+
 } // namespace Mu
 
 /**