add rx80 skeleton

[goldnchild]

Here's a skeleton for the epson rx80.

[cuavas]

• Please turn this into a “private implementation” device in an anonymous namespace, like e.g. all the newer Apple II cards.
• Please just use .set_constant(...) for the inputs that need to see something other than the default value and leave the outputs unconnected for now, since this is really just a skeleton to allow the ROM to be debugged.