Skiplist : doc improvement

[lyrm]

Changes

• Update documentation to match other data structures
• Add examples

Potential TODO (can be in a future PR)

As the hash table, the functions compare_and_set, compare_and_remove and try_set would be quite useful.

• find_exn

Note

• of_list can only be implemented as a List.iter (fun k v -> try_push t k v |> ignore) l so not really interesting
• try_add_all can't be done atomically

In a future PR

• try_set
• compare_and_remove
• compare_and_set