Support Rust returning -> Result<_, String>

[brittlewis12]

Support Rust returning -> Result<_, String>, e.g.:

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        fn do_fallible_work() -> Result<(), String>;
    }
}

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Incorporate the changes from #282 to implement swift's Error protocol for ruststring, & extend them with the tests requested in #295 (comment)

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thanks for your work on this lovely crate!

[chinedufn]

Looks good. Minor feedback, then we can land this. Thank you for working on this.

[chinedufn]

Please update your PR body with an example of a simple bridge module that is now possible.

This example will be used in the release notes.

Can just be a bridge module with a simple function that returns -> Result<String, String>

[chinedufn]

Looks great. Thank you for your first contribution. I'll merge once tests pass.