Support throwing initializers

[NiwakaDev]

This PR implements throwing initializers. See: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/errorhandling/.

Here's an example of using this feature:

// Rust
#[swift_bridge::bridge]
mod ffi {
    enum ResultTransparentEnum {
        NamedField { data: i32 },
        UnnamedFields(u8, String),
        NoFields,
    }
    extern "Rust" {
        type ThrowingInitializer;
        #[swift_bridge(init)]
        fn new(succeed: bool) -> Result<ThrowingInitializer, ResultTransparentEnum>;
        fn val(&self) -> i32;
    }
}

// Swift
do {
    let throwingInitializer = try ThrowingInitializer(false)
} catch let error as ResultTransparentEnum {
    //...
} catch {
    //...
}

[chinedufn]

Great work on this. Looks good. Left some minor feedback.

[NiwakaDev]

@chinedufn Done.

[chinedufn]

Looks great thanks.