Support awaiting messages

[philpax]

At present, Messages require you to set up a subscription that lasts until you cancel it, which is not the most ergonomic for one-shot messages or for a back-and-forth protocol.

Instead, we should

• offer the ability to one-shot subscribe to a message (this existed previously, but was removed for Reasons:tm:)
• use the former to provide a nice async wrapper for awaiting messages:

let (source, value) = MyCoolMessage::receive().await;

(it feels to me that (value, source) is a better order, but (source, value) is consistent with subscribe)

Also relevant:

• A consistent way to cancel subscriptions from within the subscription #673
• Implement RPCs with responses #253

[chaosprint]

Is this related to #665

[philpax]

Nah, this is more of an API detail about how you use messages than how the messages are sent.