Prettify sly-stickers UI slightly

* contrib/sly-stickers.el (sly-stickers--recording-pretty-description): New helper.
(sly-stickers--populate-sticker, sly-stickers--replay-prompt): Use it.
(sly-stickers--recording): Store `value-descriptions` slot, not `description'.
(sly-stickers--make-recording): Use `value-descriptions'
(sly-stickers--replay-help): Add quit help.
(sly-stickers-replay, sly-stickers-fetch):

* contrib/slynk-stickers.lisp (recording-description-string): Fix bug.
(describe-recording-for-emacs): Check for LISTP-ness of values slot.