[kernel] Simplify mailbox code, and messages

A number of simplifications of mailboxes now that the interface is much
simpler, and synchronous.

* call and respond can now only transfer one handle at a time
* mailbox objects got rid of the message queue, and just have
  wait_queues of blocked threads, and a reply_to map.
* threads now have a message_data struct on them for use by mailboxes

src/libraries/util/util/deque.h

@@ -11,11 +11,11 @@ namespace util {
 template <typename T, unsigned N = 16>
 class deque
 {
+public:
     struct node { T items[N]; };
     using list_type = linked_list<node>;
     using node_type = typename list_type::item_type;
 
-public:
     class iterator
     {
     public: