Simplify and robustify sly-mrepl result passing to Emacs

* CONTRIBUTING.md: Update example and fix a typo.

* contrib/sly-mrepl.el (sly-mrepl--result-counter): Remove variable.
(sly-mrepl-mode): Don't set `sly-mrepl--result-counter'
(:write-values): Rename arg to "results".
(:clear-repl-history): Don't set `sly-mrepl--result-counter'.
(sly-mrepl--eval-for-repl): Rewrite using `cl-destructuring-bind'.
(sly-mrepl--make-result-button): Only two arguments.
(sly-mrepl--insert-returned-values): Don't use `sly-mrepl--result-counter'.
(sly-mrepl-shortcut-alist): Add "clear repl" as a REPL shortcut.
* test/sly-stickers-tests.el (stickers-in-a-file): New test.
(stickers-record-stuff): Fix FIXME.

* contrib/slynk-mrepl.lisp (make-results): New helper.
(mrepl-eval): Use MAKE-RESULTS.
(inspect-entry): Don't use SLYNK::TO-LINE here.
(copy-to-repl): Use MAKE-RESULTS.