Stabilize spawning an asynchronous task and convenience methods on ThreadPool

[nikomatsakis]

Building on #368, this branch stabilizes the spawn API and a bunch of convenience methods in ThreadPool:

• rayon_core::ThreadPool::join
• rayon_core::ThreadPool::scope
• rayon_core::ThreadPool::spawn
• rayon_core::spawn -- runs async task in current (or global) thread-pool

The only one that enables something fundamentally new that wasn't available in a stable API is rayon_core::spawn. The ThreadPool APIs are convenience wrappers around other APIs.

The following APIs remain unstable:

• ThreadPool::global -- I don't know that we want to expose th Arc
  here, although it's no real commitment
• everything related to futures -- that is all planned to change

[cuviper]

I have questions about #368 which you stacked this on, but I'm happy with the selected stabilizations.

ThreadPool::global -- I don't know that we want to expose the Arc here, although it's no real commitment

It's being &'static Arc that's weird to me. I don't think we have any APIs needing Arc<ThreadPool>, and 'static already proves it won't go away. (I've forgotten the reason why this is even Arc at all...)

[nikomatsakis]

I've forgotten the reason why this is even Arc at all...

I'm not sure the reason is super good. I think it was because I found that some code I was using wanted to have an Arc<ThreadPool> so that it owned the thread-pool, and this way that thread-pool could be either locally created or the global one. I predicted that using Arc would be a common choice because it's handy to share the thread-pool widely.

But maybe just returning &'static ThreadPool is better, and people can use Cow or something to accommodate the "maybe shared" case (but Cow doesn't play nice with Arc...).

[seanmonstar]

I haven't tested yet, but wouldn't a crate be able to automatically add this cfg in using a build.rs, without consumers knowing?

[cuviper]

@seanmonstar I believe that can only enable a cfg for that crate itself, not its build of rayon.

[nikomatsakis]

@seanmonstar I don't know, maybe. If so, I mean, what ya' gonna do...