This gets a swapchain displayed and fills out a number of the image
related parts of the API: image creation, binding to descriptor sets,
and blitting.
Add workgroup size to dispatch call (needed by metal). Change all fence
references to mutable for consistency.
Move backend traits to a separate file (move them out of the toplevel
namespace in preparation for the hub types going there, to make the
public API nicer).
Add a method and macro for automatically choosing shader code, and
change collatz example to generate all 3 kinds on build.
A bunch of loose ends remain, including needing to plumb the size of the
workgroup through. Image and swapchains also need to be added. But it
does run collatz.
Progress towards #95.
