break out I2C and Servo features into classes

[soundanalogous]

To help simplify maintenance of all of the StandardFirmata* variants as well as improve code readability (especially in the massive sysexCallback switch statement), break out the Servo, I2C and Serial features into classes. This also enables conditionally including these features so that StandardFirmataEthernet and StandardFirmataEthernetPlus can be merged (user comments or uncomments Serial feature include to add or remove the extra functionality) - in the same way, Serial and other new functionality can be conditionally added to StandardFirmataWiFi, StandardFirmataBLE, etc.

The only downside is this will increase the memory usage somewhat.

[soundanalogous]

start with Serial

[soundanalogous]

going to try to get this into 2.6 to maintain sanity

[soundanalogous]

Made good progress on this today for Serial: https://github.com/firmata/arduino/pull/268.

[soundanalogous]

Serial is merged. Changing issue name to reflect work remaining.