Add support for opaque Copy types

[chinedufn]

This commit introduces a #[swift_bridge(Copy(...))] attribute that can
be used to indicate that an opaque Rust type should be passed across the
FFI boundary via a copy instead of by boxing it.

For example, the following is now possible:

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        #[swift_bridge(Copy(16))]
        type UserId;

        #[swift_bridge(Copy(16))]
        type OrganizationId;
    }
}

use uuid:Uuid;

#[derive(Copy)]
struct UserId(Uuid);

#[derive(Copy)]
struct OrganizationId(Uuid);