v2.0.0

This is a major update to Circuits.GPIO that modernizes the API, restricts usage
to Nerves and Linux, and updates the Linux/Nerves backend to the Linux GPIO cdev
interface.

It is mostly backwards compatible with Circuits.GPIO v1. Please see PORTING.md
for upgrade instructions.

• New features

  • Support alternative backends for different operating systems or for
    simulated hardware. The Linux cdev backend can be compiled out.

  • Circuits.GPIO.open/3 is much more flexible in how GPIOs are identified.
    Specifying GPIOs by number still works, but it's now possible to specify
    GPIOs by string labels and by tuples that contain the GPIO controller name
    and index. See t:gpio_spec/0 and the README.md for details.

  • List out available GPIOs with Circuits.GPIO.enumerate/0. Other helper
    functions are available for getting more information about each GPIO too.

  • Specify pull modes in general rather than only Raspberry Pis on Linux and
    Nerves

  • Easily do one-off reads and writes with Circuits.GPIO.read_one/2 and
    Circuits.GPIO.write_one/3

  • Improved performance on Nerves and Linux; kernel-applied timestamping of
    GPIO input events

  • Add Circuits.GPIO.Diagnostics to automate runtime testing

• Changes

  • More consistent error returns. Unexpected errors return {:errno, value}
    tuples to help correlate errors to low level docs
  • Deferred loading of the NIF to simplify debugging of GPIO backends.
    Segfaults crash on first use of Circuits.GPIO rather than on load.