Refactor MODEM code

[tschak909]

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The current WiFi MODEM code lives monolithically inside sio/modem.cpp.

This should be re-factored outward into its own library under lib/ so it can be used by other peripheral buses (and by CP/M)

The idea here is to:

• Seperate out the core modem code
• Call an initialization function with a pointer to the desired UART.
• Replace explicit calls to fnUARTSIO to use this pointer.
• Make lib/sio/modem.cpp use the now factored library.

[brad-colbert]

Is this simply a "move and create an interface" job?

[tschak909]

yeah.