Support returning Result<(), TransparentEnum> from Rust async functions.

[NiwakaDev]

This PR adds support for returning Result<(), TransparentEnum> from Rust async functions.

Here's an example of using this:

//Rust
#[swift_bridge::bridge]
mod ffi {
    enum SomeEnum {
       //...
    }
    extern "Rust" {
        async fn some_function() -> Result<(), SomeEnum>;
    }
}

//Swift
do {
    let value = try await some_function()
    //...
} catch let error as SomeEnum {
    //...
} catch {
    //...
}

[NiwakaDev]

Note that This PR doesn't support returning Result<TransparentEnum, ()> from Rust async functions. However, I guess that nobody would use it. If needed, I'll add it to this PR.

[NiwakaDev]

If this is merged, I guess that we can implement #168.

[chinedufn]

However, I guess that nobody would use returning Result<TransparentEnum, ()> from Rust async functions.

Yeah I totally agree.

It's unlikely that someone will run into this signature and if they do they can open an issue and use #[swift_bridge(return_with = ...)] to convert it into an Option<TransparentEnum> in the meantime.

If this is merged, I guess that we can implement #168.

Yeah!! Then we can finally address this and make it a Result<Option<TransparentStruct>, reqwest::Error>> or something like that

swift-bridge/examples/async-functions/src/lib.rs

Lines 15 to 17 in 72c2fa5

	// TODO: Return a `Result<MyIpAddress, SomeErrorType>`
	// Once we support returning Result from an async function.
	async fn get_my_ip_from_rust() -> ffi::MyIpAddress {

[chinedufn]

Awesome. Very nice. Thanks for implementing this.

Looks like your custom result implementation is holding up well since adding this support only took like 3 extra lines of code (excluding all of the tests),

Nice!