Add "pretty-print" and "describe" to trace-dialog parts

* contrib/sly-trace-dialog.el (sly-trace-dialog-part): Add two new
button actions.

* contrib/slynk-trace-dialog.lisp (pprint-trace-part)
(describe-trace-part): New slyfuns.
(instrument): Don't need this setf expander.
WIP sly-stickers contribs

Still some way to go, but already workable.

* sly.el (sly-compilation-position): Factor out of sly-compile-string.
(sly-after-compile-functions): New variable.
(sly-before-compile-functions): Change semantics slightly.
(sly-compile-file): Call with current buffer.
(sly-compile-region): Pass hook to `sly-compile-string' so
`sly-after-compile-functions' can be called.

* slynk.asd (:slynk-stickers): Add system.

* contrib/sly-stickers.el (sly-stickers): Add slynk-dependencies.
(sly-stickers-mode-map): Use C-c C-s as a prefix map.
(sly-stickers--stickers, sly-stickers--sticker-keymap)
(sly-stickers--make-sticker, sly-stickers--sticker-id)
(sly-stickers--arm-sticker, sly-stickers--disarm-sticker)
(sly-stickers--populate-sticker)
(sly-stickers--bring-out-your-dead): New.
(sly-stickers-dwim): Use `sly-stickers--make-sticker'
(sly-stickers-fetch): New function.
(sly-stickers-commit-stickers): Redesign.

* contrib/sly-stickers.el (sly-stickers-mouse-hover-face): New face.
sly-stickers uses sly-buttons like everything else

* contrib/sly-stickers.el (sly-stickers-mouse-hover-face): Don't need this.
(sly-stickers-mode-map): Bind "C-c C-s S" to `sly-stickers--fetch'
(sly-stickers--stickers-between): New.
(sly-stickers--sticker): New button type.
(sly-stickers--set-face): New function.
(sly-stickers--sticker): Create a sticker button.
(sly-stickers--arm-sticker, sly-stickers--sticker-id)
(sly-stickers--disarm-sticker, sly-stickers--populate-sticker): Use buttons.
(sly-stickers--decrease-prio, sly-stickers--increase-prio): New
fucntions.
(sly-stickers--sticker-priority): New function.
(sly-stickers--delete): Improve to understand priorities.
(sly-stickers--sticker-modified): New hook.
(sly-stickers-operate-on-sticker): The "operate on existing
sticker" part is taken to the sticker's own keymap.
(sly-stickers-fetch): Don't warn too much.
(sly-stickers-commit-stickers): Ensure current package.
(sly-stickers-commit-stickers): Ensure a good sorting.