* contrib/sly-stickers.el (sly-stickers--stickers-between)
(sly-stickers--stickers-at, sly-stickers--stickers-between)
(sly-stickers--stickers-exactly-at): Resurrected abstractions.
(sly-stickers--sticker, sly-stickers--sticker-substickers)
(sly-stickers-clear-region-stickers)
(sly-stickers-delete-sticker-at-point)
(sly-stickers-maybe-add-sticker)
(sly-stickers-compile-region-aware-of-stickers)
(sly-stickers-after-buffer-compilation): Use sticker-specific abstractions.

* lib/sly-buttons.el (sly-button--overlays-in)
(sly-button--overlays-between, sly-button--overlays-exactly-at)
(sly-button--overlays-at): Take optional `filter' arg.
(sly-button--searchable-buttons-at): Use `sly-button--overlays-at'.