Support std::future

[davidbarsky]

This is pretty important for a 0.1 tracing release.

[hawkw]

This isn't necessary for tracing v0.1, as it doesn't actually touch the tracing crate --- this change would be in tracing-futures. I'd definitely call it a release blocker for that crate though!

[mbilker]

I sort of have this working in a local build that I did awhile ago. I'll have to see if I can get it working again and see if I can shave off the need to have the type be Instrumented<Pin<Box<T>> to get Unpin.

[mbilker]

I put up my current work over at https://github.com/mbilker/tracing/tree/std-future. Currently, you have to call boxed_instrument when working with std::future so the inner future gets pinned.

[davidbarsky]

With the merging of #40, I think we can close this issue now!