First sketch: Rust backend

[calebbuffa]

Description

This is purely for discussion and collaboration related to #979. My hope is to get feedback and iterate on what a polyglot implementation might look like. For this first iteration, I replaced async++ with a Rust backed async crate exposed by the C FFI. The hope is that this repo gains the memory safety of Rust, while also beginning a larger effort to expose this libraries excellent algorithms to other languages.

Issue number or link

See #979

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

Yes, I would like feedback on what the community thinks of this approach. I would like to iterate and improve on the currnt implementation, and again would apprecate feedback on this.

Testing plan

This merely replaced the backend to CesiumAsync. Tests were added, but alle existing tests also test this change.

Reviewer checklist

Thank you for taking the time to review this PR. By approving a PR you are taking as much responsibility for these changes as the author.

As you review, please go through the checklist below:

• Review and run all parts of the test plan on this branch and verify it matches expectations.
  • If the issue is a bug please make sure you can reproduce the bug in the main branch and then checkout this branch to make sure it actually solved the issue.
• Review the code and make sure you do not have any remaining questions or concerns. You should understand the code change and the chosen approach. If you are not confident or have doubts about the code, please do not hesitate to ask questions.
  • Code should follow the C++ Style Guide
• Review the unit tests and make sure there are no missing tests or edge cases.
• Review documentation changes and updates to CHANGES.md to make sure they accurately cover the work in this PR.
• Verify that the Contributor License Agreement has been submitted, if needed.