irc: Convert handle-line to a message handler.

irc: Convert handle-line to a message handler.

* 8sync/systems/irc.scm (<irc-bot>): Add 'handle-line action handler,
which points to handle-line generic.
(dispatch-raw-line): Send message to our own actor to call handle-line.
(handle-line): Convert to be an action handler.

* demos/ircbot.scm:
* doc/8sync-new-manual.org: Update implementers of handle-line generic
method.