Platform: AVR: Remove core files.
Communication: Rework I2C enums, namespaces and delegate, master and adapter classes.

Remove i2c namespace for a "flatter" approach.
Move I2C delegate and adapters to peripheral/.
Convert all enums into enum classes.
Merge all adapters into one file.
Add initializer with Datarate, not complex divisors.
Peripheral: I2C: Move and adapt Xmega I2cMaster to peripheral/i2c/xmega/.
Peripheral: GPIO: Move set(status) implementation into interface.
Typos.
Add {{ generation_block }} to some templates.