Enum BastionPathElement Copy item path

Bastion::children(|children| {
    children.with_exec(|ctx: BastionContext| {
        async move {
            ctx.tell(&ctx.signature(), "Hello to myself");
             
            let msg: SignedMessage = ctx.recv().await?;
            let elem: &Option<BastionPathElement> = msg.signature().path().elem();
            assert!(elem.is_some());
            assert_eq!(elem, ctx.signature().path().elem());

            Ok(())
        }
    })
}).expect("Couldn't create the children group.");