Support calling async Swift throwing functions from Rust via Result<(), E>

[Choochmeque]

• Add support for calling async Swift functions that throw errors from Rust and return void
• Swift throwing functions are mapped to Rust Result<(), E> return type

Example

// rust
#[swift_bridge::bridge]
mod ffi {
    enum SwiftError {
        SomeError,
    }

    extern "Swift" {
        async fn swift_throwing_function_void(succeed: Bool) -> Result<(), SwiftError>;
    }
}

// swift
func swift_throwing_function_void(succeed: Bool) throws(SwiftError) {
    if !succeed {
        throw SwiftError.SomeError
    }
}

[Choochmeque]

Hi @chinedufn,
added support for Result<(), E> for async functions too, could you please review it when you have time?

[Choochmeque]

@chinedufn, could you please review this PR as well?