actors; Move message definitions earlier in the file.

The original positioning would mean that sometimes the accessors would
break in some of the actor/hive methods.  Defining the messages before
any methods make use of them seems to fix the problem.

* 8sync/systems/actors.scm (<message>, message?, make-message-intern)
  (message-id, message-to message-from, message-action, message-body)
  (message-in-reply-to, message-replied, set-message-replied!)
  (message-deferred-reply, set-message-deferred-reply!, make-message)
  (%nothing-provided, message-ref, kwarg-list-to-alist, send-message)
  (send-message-wait, reply-message, reply-message-wait):
  Moved location to earlier in file.