Added "message" scope to supported scopes.
Invoke "before" and "after" hooks around the respective handlers invocations.
Extracted helper function for prefixing keywords.
Use a shorter namespace alias.
Extracted installation of scope expiration slots to helper function.
(1) Extracted functions for installation of scope slots (expiration, hooks).
(2) BUG FIX: Store and return the handler's return value.
Added function to send expiration message for the "message" scope.

...for the sake of completeness.
Move any hook slots (before-..., after-...) from the kwargs to the ":slots"
map.

This is for caller convenience: hook slots can be specified directly or in
the slots map. The former, if any, will then simply be moved.
Added resorce support to the message handler.
Removed the handler from the arguments for the handler's hooks.