* sly.el (sly-dispatch-event): Group channel-related
events. Accept new `:invalid-channel' event.
(sly-close-channel): Improve error message.

* swank.lisp (:swank): Export SWANK:STOP-PROCESSING symbol.
(process-requests): Catch SWANK:STOP-PROCESSING tag.
(thread-for-evaluation): Shoosh compilers.
(dispatch-event): Handle invalid channels.
(close-channel): New function.
(channel-send): Document generic.

* contrib/sly-mrepl.el (sly-mrepl-mode): Don't add to `kill-buffer-hook' here.
(sly-mrepl-new): Rather here when we're sure to have a remote channel.
(sly-mrepl--delete-process): Delete it.
(sly-mrepl--teardown): Delete process, teardown remote, close local.

* contrib/swank-mrepl.lisp (:compile-toplevel): import SWANK:CLOSE-CHANNEL
and SWANK:STOP-PROCESSING.
(listener-channel): Use accessors.
(initialize-instance): Fix whitespace.
(create-mrepl): Give channel a name
(drop-unprocessed-events): Add docstring.
(spawn-listener-thread): Learn to teardown thread.
(:teardown): New channel method.
(mrepl-eval): Rewrite env-setting for clarity.