Make the hub abstraction connect to the mux, rather than directly to the
Vulkan back-end.
As of this commit, both command line and winit examples work (on
Vulkan). In theory it should be possible to get them working on Dx12 as
well by translating the shader code, but there's a lot that can go
wrong.
This commit also contains a bunch of changes to mux to make conditional
compilation of match arms work, and new methods to support swapchain.
Adds a new "mux" module which can have multiple backends. As of this
commit, it's not wired up at all, but the functionality should be
reasonably complete.
Minor tweaks to the backend trait to accommodate this, mostly changing
Fence and Semaphore to references so they don't need to be Copy.
Part of the work toward #95
