Rust: finish and public crossbeam-skiplist

[brson]

In https://github.com/pingcap/talent-plan/pull/138/files we are probably going to strongly suggest using the crossbeam skiplist.

This crate isn't finished nor published yet though.

Per @stjepang the code is done though. The only things left to do are write tests and docs.

That's something we can do.

After getting crossbeam-skiplist published, update any text about it to remove any discussion about its incompleteness, git deps, etc. Update the example projects to use the published version.