Add spawn function

[danog]

No description provided.

[kelunik]

As previously discussed, I don't see any benefit over using async().

[danog]

One very clear benefit is to avoid the overhead of creating a future, without having to use EventLoop::queue.

[trowski]

The main purpose I see to this is providing a tool to create a fiber without the future within the Amp eco-system. Otherwise obviously it is exactly the same as using EventLoop::queue(). I can see benefits to this, as it might be easier for newcomers to discover Amp\spawn() over needing to read the event loop docs.

For a full compliment of Amp namespaced functions, we'd probably also need Amp\setEventLoopErrorHandler() and Amp\runEventLoop() (or similar). Maybe I've forgotten something? I think without these as well, there's not much benefit because users will still need to learn about the event loop API.