Vulkan backend

[athas]

Vulkan is increasingly everywhere and it does support compute shaders. We did a prototype backend once, but never polished it enough for general usage (and I recall Vulkan compute shaders were a bit immature). We need to return to this.

[athas]

@Zyntax9 is the one who did the Vulkan backend. The compiler has probably diverged too much for his old code to be useful, perhaps except for the SPIR-V generator, but his report still contains useful lessons.