Preparatory work for sly-stickers buffer compilation support

* sly.el (sly-temp-message): New function.
(sly-compilation-finished-hook): Default value is now a list.

* contrib/sly-stickers.el (sly-stickers): Add to `sly-compilation-finished-hook'
(sly-stickers--disarm-sticker): Disarmed sticker perhaps was never pre-armed.
(sly-stickers-after-buffer-compilation): New function.