Migrate to the Velo handle in 0.21

Velo is now a cheap handle. VeloBuilder::build() returns Velo. It used to return Arc<Velo>.

Velo is Clone. A clone adds one reference count and does not copy any state. Every clone is the same instance.

How to migrate

• Replace Arc<Velo> with Velo in your types and signatures.
• Replace Weak<Velo> with WeakVelo in your types and signatures.
• Remove velo.as_ref() and &*velo. Pass &velo instead. Replace velo.as_ref().clone() with velo.clone().
• Keep .clone() as it is. It now clones the handle.
• Change Arc::clone(&velo) to velo.clone().
• Change Arc::downgrade(&velo) to velo.downgrade(). It returns a WeakVelo, and WeakVelo::upgrade() returns Option<Velo>. A handler that reaches its own instance must hold a WeakVelo. A Velo clone in the handler keeps the instance alive, so its final drop never runs.
• Change Weak::new() to WeakVelo::default(). It points at no instance, and upgrade() returns None.
• Change Arc::ptr_eq(&a, &b) to a.instance_id() == b.instance_id(). Clones of one instance share an InstanceId, and separate instances never do. Velo does not expose reference counts.
• The other Arc and Weak methods have no counterpart: try_unwrap, into_inner, get_mut, make_mut, strong_count, weak_count, and Weak::ptr_eq. For identity, compare instance_id(). To share, clone.
• Remove Arc::new(velo) if you wrapped a built Velo. Pass velo or velo.clone() instead.

// 0.20: `velo` is an `Arc<Velo>`
let velo = Velo::builder().add_transport(transport).build().await?;
let worker = Arc::clone(&velo);

// 0.21: `velo` is a Velo
let velo = Velo::builder().add_transport(transport).build().await?;
let worker = velo.clone();

Arc<Messenger> and the other Arc values that Velo hands out do not change.

What does not change

• Clones share one instance and one set of handlers, streams, and rendezvous state.
• The final drop of the last clone cancels the streaming services. Dropping any other clone does not.
• graceful_shutdown runs its teardown once, even when you call it through different clones. A later call waits for the first and then returns, unless the first teardown failed. Then a later call panics at once.
• shutdown calls graceful_shutdown, then closes the streaming services. It does the streaming step again on each call.
• Wire formats and velo-ext traits do not change.