Implement an experimental VRAM to TTY bridge
Polish VRAM access to TTY TX emulation

It assumes Apple BASIC's manner of controlling screen.
Apple BASIC may be able to work exactly like running through
a virtual terminal device.
Now GPIO can be accessible via Apple BASIC

POKE -16240, 255 # Set all bits of GPIO0 as output ports
POKE -16224, 255 # Output all one to GPIO0
POKE -16224, 0   # Output all zero to GPIO0