irc: Move irc-bot code to make use of generic methods

irc: Move irc-bot code to make use of generic methods

ircbot.scm demo also now makes use of subclassing.

* 8sync/systems/irc.scm (<irc-bot>): Removed line-handler field.
(irc-bot-dispatch-line): Renamed from irc-bot-dispatch-raw-line.
All callers changed.
(irc-bot-send-line): Moved position in file.
(irc-bot-handle-line): New method.
(irc-bot-handle-misc-input, irc-bot-handle-user-join)
(irc-bot-handle-user-quit): New stub methods.

* demos/ircbot.scm (<my-irc-bot>): New variable.
(irc-bot-handle-line): Now a generic method extending same named
method in 8sync/systems/irc.scm.  Previously was `handle-line'.
(run-bot): Use <my-irc-bot>.
(main): Remove debugging pk.