What kinds of devices to support?

[abrown]

Initially, the kinds of devices that can provide parallelism is rather limited but the intent is to support any kind of device that can execute a parallel for:

(typename $device_kind
  (enum (@witx tag u8)
    $CPU
    $DISCRETE_GPU
    $INTEGRATED_GPU
  )
)

What variants should be visible here? Options could include merging the *GPU variants, adding FPGA, adding the generic ACCELERATOR, etc.