USB communication with the host

[stepansnigirev]

Working on https://github.com/cryptoadvance/specter-diy/issues/31

Changes:

• added unixport folder that partially implements pyb module for simulator:
  • Pin that prints instead of blinking
  • virtual UART on a TCP socket - you can connect to it using telnet for example.
  • virtual USB_VCP on a TCP socket
• qrscanner now uses the same code for simulator and for the board
• usbhost class that can trigger callback on a new line
• boot.py file that redirects REPL to STLink port instead of normal USB

Communication over USB happens in a human-readable form, because why not. Every command should end with \r or \r\n

USB commands (to make it fully compatible with HWI):

• [x] fingerprint - returns fingerprint of the root key (4 bytes in hex representation)
• [x] xpub <derivation_path> - returns xpub with corresponding derivation and shows it on the screen of the device. Example: xpub m/84h/1h/0h
• [x] sign <psbt>
• [x] showaddr <format:pkh/wpkh/sh-wpkh> <derivation>
• [x] importwallet <descriptor>

Would be useful to implement later, but I would postpone it for another pull request:

• showdescraddr <descriptor>
• signmessage <message>

Also todo: update documentation