WIP: Allow the REPL/console to be mirrored onto a UART

[jepler]

This is certainly in a "hacky-hack" stage. It

• seems to work on nRF52840
• almost certainly fails to build on every other platform
• takes some liberties with existing HAL APIs and memory allocations
  but it does achieve the goal of letting you write in boot.py

import busio, board
busio.UART.make_console(board.TX, board.RX)

to get the regular REPL and program output mirrored both on USB (if attached) and via UART.

The platform build problem is of course surmountable; but can the "hack factor" of the specific implementations be reduced enough that this is acceptable for core? It will be good enough for my specific purposes, getting a CircuitPython REPL into a Commodore 64 via its serial port as a proof of concept.

[tannewt]

I'd prefer it if there were two additional functions on supervisor .add_serial_in() and .add_serial_out() that way we can separate the two things. A keyboard for example would only be serial_in. We probably also want a forever kwarg that keeps the object alive past the vm. Folks may want to redirect temporarily too.

Instead of needing a separate hook, you can probably reuse the stream api stuff.

Also, we may want to formalize a way to "save" certain heap state at the end of a vm to preserve it for longer.

[jepler]

I'm going to go ahead and close this up but leave the branch in my repo. This approach isn't going to generalize, and more design is needed.