Implement a LaserWriter IISC printer emulation

[demik]

Hello,

The protocol has been mostly reversed, so here is a PR to implement it in Snow.
There is probably some stuff to improve, but as a first shot I hope this isn't too bad

Thank you for the nice architecture. The SCSI code is really nice, it was easy to implement it. (I did hit a couple of bugs, which were fixed last week)

Couple of notes:

• Implementation was based ethernet.rs, then the rest was a fight with rustc (can't do OOP to save my life)
• printed pages are saved like screenshots, except with "Snow print" instead of "Snow screenshot"
• added SNOW_SCSI_TRACE_PRINTER to ease future debugging
• I think the rendering is done inside the emulation thread, not sure this is right

If this is ok, I would like to add a few more things, for example use the nice debug features of Snow (debug paper size, count pages or commands, etc)

Edit: the printer feature needs --feature printer, not sure how that's gonna work with the CI

[demik]

I fail to understand the MINGW64 build error. GitHub acting again ?