Gen 2 update: stacks n keyboards.

SBTCVM's CPU makes some progress, as the cpu is now equipped with a whopping 2 stacks. 

Along with the cpu, SBTCVM Gen 2-9 is now capable of interactive programs, thanks to a new TTY Input device, that allows for a basic serial text interface.

Aside from the usual assembly enhancements, SSTNPL has gotten its share of changes too:

•  you can now create variables based on character code values.
•  you can read from the tty input buffer into a variable for easy parsing of keystrokes.
• return is now stackable.
• separate subroutine gotos are required to use return.
• conditional returns are now available.