Change `Device` to `Backend` or something similar ?

[mwarusz]

The motivation is that there could be different backends for the same device, e.g. a single threaded and multithreaded backend for CPUs. The current usage of Device looks to me like it is really an abstraction for backend. Ideally, those concepts should be separated. Moreover, currently it is true that

julia> CPU <: Device
true
julia> GPUifyLoops.isdevice(CPU())
false

which does not seem great.