Add support for crossbeam

If somebody actually takes on this work then this issue should probably be split up.

Previous work on this: https://github.com/awslabs/shuttle/pull/94.

Creating this issue in order to close that PR as it has been around for 3 years and it doesn't look like anyone is going to drive it to completion.

Start by reviewing the previous work in PR #94 and determine which parts of crossbeam support remain. Split the work into concrete pieces, then define completion for each piece before implementation; the issue is done when the agreed crossbeam support is complete and the corresponding checks pass.

Written by the indexing model from the issue text.