lib: update mu-contact-{fields,contact,flags,priority}

Improve cohesion and tests.

lib/mu-message-contact.hh

@@ -26,10 +26,11 @@
 #include <functional>
 #include <cctype>
 #include <cstring>
-
 #include <cstdlib>
 #include <ctime>
 
+#include "mu-message-fields.hh"
+
 struct _InternetAddressList;
 
 namespace Mu {
@@ -125,6 +126,27 @@ struct MessageContact {
 		return  cached_hash;
 	}
 
+
+	/**
+	 * Get the MessageField for this contact-type, if any.
+	 *
+	 * @return the message-field or nullopt.
+	 */
+	constexpr std::optional<MessageField> const field() {
+		switch(type){
+		case Type::From:
+			return message_field(MessageField::Id::From);
+		case Type::To:
+			return message_field(MessageField::Id::To);
+		case Type::Cc:
+			return message_field(MessageField::Id::Cc);
+		case Type::Bcc:
+			return message_field(MessageField::Id::Bcc);
+		default:
+			return std::nullopt;
+		}
+	}
+
 	/*
 	 * data members
 	 */