* sly-mrepl.el: Nearly organize into sections
Add output-clearing commands to sly-mrepl

* contrib/sly-mrepl.el (:clear-repl-history):  New channel method.
(sly-mrepl--insert-output): Propertize with field.
(sly-mrepl--assert-mrepl): New helper.
(sly-mrepl-insert-input): Use `sly-mrepl--assert-mrepl'
(sly-mrepl-clear-repl, sly-mrepl-clear-recent-output): New
interactive commands.
(sly-mrepl--menu): Add new commands to menu.

* contrib/swank-mrepl.lisp (:clear-repl-history): New channel method.