Fixed comments in Array.
Changed Tracer to take const Array of rays.  Misc code cleanups.
Refactored RingBuffer and JobQueue classes.

There is now a RingBufferConcurrent class, which JobQueue uses.
This simplified the code of JobQueue substantially, and allows
the use of concurrent ring buffers in other contexts.
Added a Timer class, for reporting timing (e.g. render time, etc.).