Changes

Refactor skiplist to avoid UB casts (#9660)

This patch removes casts between struct skiplist * and struct skipcell *, and removes the "layout compatibility" fields in skiplist that were there to enable these casts.

The only algorithmic difference is that caml_skiplist_find is now "stop-at" (see discussion here), as this was slightly easier to write in the no-cast style.
 (commit: 5946b93)