gpio: Create gpio.Group interface

[gsexton]

Fixes #30

The idea behind this interface is to give users a way to atomically modify multiple GPIO pins in one transaction.

This is essentially the interface for the LineSet I created for the IOCTL based GPIO. I'll have another PR coming to make the gpioioctl LineSet implement this interface.

I'll be adding this interface to the MCP23008, and implementing it in a future driver for a CP-2112, and MCP2201, etc. Using this Group() will make writing GPIO Backpacks for things like LCDs trivial. We can have one HD44780 GPIOBackpack that takes a Group, along with a reset and Enable pin.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.7%. Comparing base (e01555d) to head (ab96a60).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #43   +/-   ##
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  Coverage   93.7%   93.7%           
=====================================
  Files         34      34           
  Lines       3930    3930           
=====================================
  Hits        3681    3681           
  Misses       244     244           
  Partials       5       5           

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[maruel]

Sounds good, thanks! I wanted to design that for years (literally) for the case of ft232h but never got around it. This looks good.