v2.0.0

This is a major update to Circuits.I2C that removes the requirement to use
Nerves or Linux. The API is almost the same and the default is to compile and
use the Linux backend, so changes may not be needed.

• Changes
  • Support alternative backends for different operating systems or for
    simulated hardware
  • For the Linux NIF, support passing a :retry count to Circuits.I2C.open/2
    that will apply to all I2C operations. This simplifies dealing with devices
    with flaky connections.
  • Defer loading the Linux NIF until Circuits.I2C.open/2 is called
  • Return error on Circuits.I2C.open/2 if the device isn't a real I2C device.
    This was previously detected on the first operation.
  • For the Linux NIF, return errno numbers rather than their strerr strings
    when they're unknown. It turned out that the strings could be more confusing
    that the numbers.