Add example using gfx-hal or wgpu

[repi]

In addition to the existing example using ash, which is quite explicit and low-level with Vulkan, it would also be very nice to have an example using gfx-hal which would be cross-platform and likely smaller and cleaner example.

@termhn has volunteered :)

[fu5ha]

gfx-hal is likely to be just about the same length, though perhaps a few less lines.

Possibly a wgpu example could be a good idea too. It uses gfx-hal internally so we get the same cross platform, but it will actually be significantly less user code complexity.

[repi]

Good idea with wgpu, that would be even higher level and nicer, and more apps and games that have started using it directly