Viaduct replacement demo

[bendk]

This shows off a potential viaduct replacement that uses new UniFFI features.

The 1000ft view is that:

• We define an HTTP backend trait in Rust (note that the method is async, via the async_trait crate)
• We can implement that trait in Rust
• or implement it in Python
• We initialize the crate with one of those backends
• Then we can make HTTP requests in Rust, independent of the implementation.

Check out the fairy-bridge README and the demo README for details. Execute run-demo.py to test it out yourself.

The UniFFI features are still a WIP. This is currently using a branch in my repo. The current plan for getting these into UniFFI main is:

• Get the 0.26.0 release out the door
• Merge PR #1818 into main
• Merge my async-trait-interfaces branch into main (probably using a few smaller PRs)

The Desktop plan needs to be explored more. I believe there should be a way to use Necko in Rust code, but that needs to be verified.

I made this using UniFFI proc-macros, to test out the technology and I have to say it was very pleasant to work with.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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[bendk]

Thanks for the review! I just updated a bunch of code based on the suggestions. I think you're right that the real interesting part will come from integrating with Necko. If the UniFFI bits are the boring part, I guess that's a good sign.