applet.program.avr: add erase command.

applet.interface.jtag_probe: flush after each REPL command. This is a great anti-frustration feature that I added to the global REPL command, but not to the applet-specific REPLs. Which led to great frustration.

applet.interface.jtag_probe: log bits in exchange XR calls.

applet.program.avr: minor fixes. NFC.

applet.video.vga_terminal: remove. The code in it is horrible, and it also uses a completely outdated version of the Boneless CPU, which will just serve to confuse.

support.task_queue: improve cancellation support. * Propagate cancellation out of poll(), wait_one(), and wait_all(). * Make cancel() an async function that waits until cancellation is finished. Before this commit, cancelling a demultiplexer read (e.g. with ^C) that was waiting on USB would cause it to fail an assert. After this commit, the read is also cancelled.

access.direct.demultiplexer: fix a few typos in comments. NFC.

cli: request terminal size in an appropriate way.

cli: use built-in Python 3.8 mechanism to handle Ctrl+C on Windows. The one in support.asignal never worked on Windows, anyway.

cli: fix path logging.