Async capability combinators

[charypar]

This PR is work in progress on #91.

It turns out, surprisingly, that async capability combinators were always possible in "user land", and could be implemented as an additional capability, but the implementation is not exactly obvious, and this type of use needs to be supported by the capabilities being orchestrated across, especially crux_http.

This PR aims to make the use of async across multiple capabilities more obvious. There are a few challenges to overcome

• The Orchestrate capability should probably come with crux_core
• Automatically derive Clone for capabilities and the Capabilities struct, to avoid various lifetime errors when moving the capabilities or the context into the async blocks - decided against, it would conflict with custom Clone implementations
• Provide a Never operation type
• Avoid producing an Effect variant for capabilities who's operations are Never () fields are #[effect(skip)]
• The use needs thorough documentation and an example
• Existing capabilities need to expose an async version of their APIs (Implement async API in crux_http #180, Implement async API in crux_time #181, Implement async API in crux_kv #182)

All feedback on the ergonomics of this is welcome at this stage, but I'd like to aim to ship this soon, ideally this week.

[wasnotrice]

@charypar thanks for this, it looks great! I'm so excited to try this in my app. I think it will help a lot with maintainability. I have some places where the flow of orchestrated events is really hard to follow.

In the example, there are two concurrent events, but it could easily be two sequential events, or any other orchestration, right?

[charypar]

@wasnotrice Yea, absolutely, it's just an open ended async context with access to the capability runtime, I'm pretty sure it can orchestrate an arbitrary graph of dependent effects (and emit events along the way).

One thing to be aware off is that it is likely to be quite hard to test "sub-steps" in isolation on long effect chains. The entire outer future is one atomic flow which can only be entered into at the top.

[StuartHarris]

yay!