examples: gpio: Refactor example

The example had organically grown and was getting hard to read, and also
had some known issues. Summary of fixes:

- Default GPIO bank and connector are now derived from the device. This
  allows this example to pass without throwing an exception on E3xx and
  X4xx series when using default arguments.
- The bitbang test is moved into its own code section, to make the rest
  more readable.
- We move all the streamer-related code into a helper struct
- Some repetitive parts of the code are moved into their own functions
- The argument --require-loopback is added, which will fail tests if
  GPIO pins are not correctly looped back externally
- --list-banks is renamed to --list_banks for consistency

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