Fix bug in assigning values to enum members
- Revamped `with-replies' to use a procedure in flow/event-loop called
  `notify-map' as its basis, moved `with-replies' to xcb/event-loop

- Slight modification to error handling code; still not ideal

- Added `create-listener' syntax, which groups event listeners
  together with procedures to stop/reset them
Expose procedures xenum-keys, xenum-values, which return the keys and
values of an xcb enum, respectively.