Implement Wire

[jgfoster]

cpp/arduino/Wire.h has a number of // TODO comments. I'd like to complete the implementation and would appreciate guidance (or at least feedback on some ideas).

One extreme would be to pull in an existing implementation with the assumption that a "real" implementation would work by reading and writing pins (pins 20 & 21 on the Duo and Mega2560). With that, it is just like any other code that reads and writes pins.

A middle ground would be to write a new implementation that reads and write pins, but that seems like duplicate work.

The other extreme would be to mock the API at a higher level (and not set pins). Tests would not look at pins but would need to call a new testing API that could get and set values at a higher level than bits on a pin. This could be something added to Godmode or to Wire itself.

I propose a new high-level API (I would want it anyway) and that it be in Wire (I'd prefer not to further pollute Godmode). Thoughts?

[ianfixes]

This refers to https://www.arduino.cc/en/Reference/Wire

The TODO items correspond to where I'd put some sort of GODMODE hook, but I wasn't sure of how much to leverage existing structures for this or write new ones -- does it override what goes on with PinHistory, or interact with #145, etc.

[jgfoster]

I expect there will have a PR on this in a couple days. Then we will have something concrete to discuss and critique.