Base: skip list in lib_base

Context

This MR duplicates the skip list data structure defined in the protocol to lib_base, as this is a generic enough structure.

We may be interested by exposing this structure in the environment to use it in the protocol instead (see !8993 (merged)).

Prerequisite for !8824 (merged).

Manually testing the MR

dune exec src/lib_base/test/main.exe -- --file test_skip_list.ml