Support failable initializers

[NiwakaDev]

Related to #235.

This PR supports failable initializers.

Something like:

// Rust side
#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        #[swift_bridge(Equatable)]
        type FailableInitType;

        #[swift_bridge(init)]
        fn new() -> Option<FailableInitType>;
    }
}

// Swift side
let failableInitType = FailableInitType()
if failableInitType == nil {
    ~
} else {
   ~
}

See: Swift Documentation - Failable Initializers

[chinedufn]

My feedback was minor, so feel free to Squash and merge after you address it.

When you Squash and merge remember to paste in your PR body as the commit message.

---

Alternatively you can rebase into a single commit, force push that, and then merge that. But Squash and merge essentially does the same thing.

[chinedufn]

Hello, hello. Still planning to land this?

[NiwakaDev]

Sorry. I'll address your review this weekend if possible.