vulkan: handle wrapping for 32-bit platforms

So, I'm torn between this approach (requiring malloc/free pair) and the approach of simply rewriting callbacks to take a uint64_t instead of a void *, at the cost of needing to have a lot of casting boilerplate everywhere else.

I'm not sure what's better, but since I don't particularly care about a tiny performance loss on 32-bit platforms, and since pointer args are overwhelmingly more common than non-dispatchable handles, I think I'll settle on this.