Serial (Zilog SCC) support

[samkusin]

Right now the SCC has enough logic to make it past startup. Onboard IIgs component backend not yet implemented. Serial communications with external devices will not work until the component backend is implemented.

Relates to the diagnostic failure reported in #72 .

• The emulator library will need an interface to read data serially from a host application.
• The library will need to request data from the host when it's ready to receive it.
• The library will need to notify the host application when data is available to send out

The emulator host will need some kind of ability to associate the serial port with an external device (i.e. USB, socket.)
For a first pass, communication via socket may be good to test terminal send/recv to the emulated machine
For a second pass, add generic USB (timing may be tricky here so socket communication is a good first pass.)

[samkusin]

Partial implementation in #116 (local loopback async, no interrupts.)