Import `kcas_data` as a part of `saturn`

[Sudha247]

kcas_data contains a nice set of compositional lock-free structures built with kcas. This is in alignment with the goal of saturn, to provide parallelism-safe data structures for OCaml 5+ users. It might be useful to expose these structures in kcas_data as a part of saturn_lockfree or even a separate sub-package.

cc @polytypic @lyrm