Able to convert OpaqueIpcMessage to "smaller" sized types.

[gatoWololo]

The following minimal example reproduces the error (I'm using the latest crates.io version of ipc-channel but I bet it works with latest commit on master as well):

use ipc_channel::ipc;
use ipc_channel::router::ROUTER;

fn main() -> Result<(), std::io::Error> {
    let (sender, receiver) = ipc::channel::<(i32, i32)>()?;
    // Use this second channel to force main thread to block until child thread
    // is done.
    let (sender2, receiver2) = ipc::channel()?;

    ROUTER.add_route(receiver.to_opaque(), Box::new(move |msg|{
        match msg.to::<i32>() {
            Ok(val) => println!("Val: {:?}", val),
            Err(e) =>  println!("Error: {:?}", e),
        };
        sender2.send(());
    }));

    sender.send((1234, 5678)).unwrap();
    receiver2.recv().unwrap();
    Ok(())
}

Which happily outputs: Val: 1234.

Basically I'm able to convert the (i32, i32) to a i32 through to::<i32>(). I only tried it for numerical types, but it seems we have observed this behavior for less trivial types: servo/servo#23818 (comment)

I'm planning to (ab)use this behavior... but it seems a little undefined-behaviory to me?