Devices: block/char devices, device files

[james-pre]

It would be nice to support devices and device files. This is more of a long-term goal, for after the 1.0 release.

Implementing devices through files would be really easy, though they obviously could not actually be written to backends. The non-persistance may be a problem.

Additionally, it would be awesome if @zenfs/dom included a "video" driver for HTMLCanvasElement