Ensure context guard is !Send

[jtescher]

Currently you can hold a ContextGuard across await points accidentally via something like:

async {
    let cx = Context::current();
    let _g = cx.attach();
    some_future.await;
    // ...
}

This may cause inconsistent and buggy behavior as ContextGuards rely on thread locals, this change causes the compiler to catch this case by marking the guard as !Send though a phantom type that is !Send.

[frigus02]

Oh nice. I remember wondering about this a while back. Looks good 👍