applet.program.avr: add support for extended addressing

applet.program.avr: fix range() while verifying memory

README: clarify.

README: update IRC link.

access.direct.demultiplexer: track upstream Windows libusb changes.

platform.rev_c{0,12}: fix typos.

platform.rev_c0: add LVDS connector pinout.

platform.rev_c12: reformat.

README: tone down I/O pin data rate expectations. 100 MHz was too optimistic and is unlikely to be achievable in most cases due to layout and component choices. The level shifters are rated up to 210 Mbps at 3V3-to-3V3, but this is the rating for just the shifters and not the complete system.

README: use `python3` to run the macOS setup script so we don't hit the system Python which is a version 2.