Move the parts of gfx-rs into this repository

[kvark]

Born in gfx-rs/gfx#3768

The rough idea here is to extract the parts of gfx-rs and move them under a sub-crate of this repository (e.g. wgpu-hal).

• the only parts needed are ones directly needed by wgpu
• everything else is to be moved to gfx-portability, which will depend on this wgpu-hal
• the API exposed by this sub-crate needs to be extra-open, so that anybody can hook into it and use it.