repl: Publish notification about repl updates to subscribers.

* 8sync/repl.scm (<repl-manager>): Convert to use define-class.
Switch all getters accessors from `repl-manager-' prefix to
dot-prefix.
Add subscribers slot.
Add action handlers for add-subscriber and remove-subscriber.
(repl-manager-cleanup, repl-manager-init): Use dot-prefix
getters/accessors.
(repl-manager-init): Update to inform subscribers after every time
we poll the REPL.
(repl-manager-add-subscriber, repl-manager-remove-subscriber): New
procedures.