Further improve sly-stickers model for recording descriptions

* contrib/sly-stickers.el (sly-stickers--populate-sticker): Change
protocol. Take a sticker and a `sly-stickers--recording'.
(sly-stickers--recording): New structure.
(sly-stickers--recording-void-p, sly-stickers--make-recording): New helper.
(sly-stickers--process-recording): Renamed from
`sly-sticker--process-sticker-description'
(sly-stickers--replay-state): Remove slots and add a `recording'
slot.
(sly-stickers--replay-prompt, sly-stickers--replay-read-binding)
(sly-stickers--replay-next-state, sly-stickers-replay)
(sly-stickers-fetch): Use new `sly-stickers--recording' structure.

* contrib/slynk-stickers.lisp (forget, fetch): Add docstring.