Using Zilog's SIO as a console

The RC2014 has an optional module called the Dual Serial Module
SIO/2 which is built around Zilog's SIO chip. This module is
nice because when paired with the Dual Clock Module and when
using port B, it's possible to run a UART with a baud rate lower
than 115200.

Collapse OS has a driver for it (although for now, only port A
is supported by it). Let's use it.

* Let's assume a xcomp unit similar to the one in
  /arch/z80/rc2014.
* Locate SIO driver in /arch/z80/rc2014/blk
* The driver main page gives you references for declarations and
  for code.
* In the base xcomp unit, replace ACIA declarations with SIO's
* Replace ACIA code with SIO's
* At the bottom, replace "ACIA$" with "SIO$".

Rebuild the binary and you're done. "(key)" and "(emit)" will
go through the SIO.