Return Result<OpaqueRust, OpaqueRust>

[chinedufn]

This commit makes it possible to return a
Result<OpaqueRust, OpaqueRust> from a Rust function.

Rust functions that return results show up on the Swift side as
functions that throw.

For example, the following is now possible:

// Rust

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        type TypeA;
        type TypeB;

        fn some_function() -> Result<TypeA, TypeB>;

        fn print_info(self: &TypeB);
    }
}

// Swift

do {
    let typeA = try some_function()
} catch let typeB as TypeB {
    typeB.print_info()
}