Allows a script to simulate a device attached via any serial protocol.
The script can read and write data and have it handled according to the
protocol as if exchanged over a serial port. The script can then do
protocol translation, data filtering and validation,
hardware-in-the-loop simulation, experimentation, etc., especially in
combination with the scripting protocol which lets the script itself
handle an attached device and so interpose any communication.
Using `luaL_Buffer` avoids the need for any heap allocation in the
common case (count <= 512 bytes) and avoids stressing out the system
heap for large reads, instead using the script heap.
Zero net flash usage change.
Adding another layer instead of just exposing UARTDriver bindings allows
substitution of the different functions for device simulation later.
Also take the opportunity to rework the docs a little.