Memory safe support for `#[swift_bridge(Copy(0))]` on zero sized types

[chinedufn]

We want to be able to support:

#[swift_bridge::bridge]
mod ffi {
    extern "Rust"{
        #[swift_bridge(Copy(0))]
        type Zst;
    }
}
struct Zst;

Not sure what happens right now.. But it's probably incorrect.. Need to investigate..

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Somewhat related to https://github.com/chinedufn/swift-bridge/issues/14