rayon-core compatibility

[dragostis]

Given the feedback, it seems clear that the next step would be to get chili to implement the rayon-core public API. This way, the crate can work as a drop-in replacement for rayon-core and we'll be able to measure performance when powering the parallel iterator API in Rayon.

Some bigger steps on the road to get there:

• get ThreadPool to hand out multiple Scopes concurrently
• add a global ThreadPool
• figure out an effective way to control heartbeat skipping from the par_iter API
• identify and implement any other API directly consumed by the par_iter API
• stub out everything else in order to make the public API identical
• benchmark par_iter on top of chili