Support Vec<OpaqueRustType> as a fn arg

[chinedufn]

For example, the following is now possible:

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

        fn rust_reflect_vec_opaque_rust_type(
            arg: Vec<ARustTypeInsideVecT>,
        ) -> Vec<ARustTypeInsideVecT>;
    }
}

---

We also fix the RustVec's deinit to only free the underlying
Vec<T> if the RustVec is owned. This prevents a double free
after passing an owned RustVec from Swift to Rust.