Switch the Vulkan backend from `gpu_alloc` to `gpu_allocator`.

[nical]

It's what we use in the D3D12 backend.

Ideally, implement Traverse-Research/gpu-allocator#235 before doing the switch to avoid changing the behavior too much.

[Elabajaba]

I've started working on this.

I've got a super stale branch from when I did this a few years ago, but I'll just redo it from scratch.