Use rayon for the raytrace-parallel example

[alexcrichton]

Currently the multithreading example we have with raytracing uses a home-grown solution for parallelization, but we should be showing off rayon!

I've got a work in progress branch for gaming this out, but there's a few notable points:

• Supporting this depends on changes to Rayon, which we're still proving out on our end.
• Currently the demo is crashing a lot due to lock contention for memory allocation on the main thread, and this is hoped to be patched in the upstream standard library
• The demo was seemingly crashing for other reasons, but I figured it'd be easier to get to the bottom of it once the allocator issue was solved.

In any case wanted to make sure we had a tracking issue for this!