Integrate DXC for translating HLSL for use in DX12. This will work
around FXC limitations and unlock the use of more advanced HLSL features
such as subgroups.
This hardcodes the use of DXIL, but it could be adapted (with a bit of
effort) to choose between DXIL and HLSL at runtime.
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.
