also move I2CFirmata to features-optional

[soundanalogous]

I've been thinking about whether or not I2CFirmata should also be moved to features-optional. The main reason for moving it would be due to the Wire library dependency (why the I2CFirmata implementation is currently all within the header file). Serial has the same issue (due to the SoftwareSerial library dependency) but is already in features-optional. When a SPI feature is eventually added it will also have a dependency issue on the SPI library. I liked the idea of features related to hardware pin functionality all being in core, but due to Arduino linker issue maybe it's best if core is really core features that don't have dependencies (everything in there now except I2CFirmata).

[finson]

I think the move would be good. Then the core is analog and digital, read and write, with no library additions and pretty much maximum memory available. That's a good solid little doo-hicky that you can do lots with right out of the box.

Shall I go ahead and move it?

[soundanalogous]

Sure. You could also move the I2CFirmata implementation back to the cpp file if you feel up to it. Otherwise I can do that later once it's merged back into ConfigurableFirmata.

[finson]

Okay.

ETA. Well, it still compiles after the move and split. Shall I do the same with ServoFirmata while we're at it?