[tracking] futures concurrency macros

[yoshuawuyts]

As per my blog post, I'd like us to try out futures concurrency macros. These should be introduced as "experimental" because they're mostly new, but I think this would make for a good starting point.

I propose these macros would live under future::* similar to task::ready in #133.

update: these macros are now available as unstable since the 0.99.7 release. Please post any feedback about it in this issue.

Macros

• future::join
• future::try_join
• future::select
• future::try_select

[yoshuawuyts]

We now have these, and I think it's okay to close this. Thanks!