actors: Add new method of running REPL via REPL actor.

actors: Add new method of running REPL via REPL actor.

Also adds new procedure to see if an actor is alive, and make use of it
in the new REPL actor.

* 8sync/actors.scm (actor-am-i-alive?): New procedure.
(ez-run-hive): Remove #:repl-server keyword.
* 8sync/repl.scm (<make-coop-server-handler, spawn-and-queue-repl-server!):
Remove.
(<repl-manager>, repl-manager-cleanup, repl-manager-init): New variables.
* demos/ircbot.scm (parse-args): Add --repl command line option.
(run-bot): Add and queue <repl-manager> actor if appropriate.