I2c resets registers on NACK (#148)

* i2c: Promote use of Deref trait

Modeled after stm32f4xx-hal's I2C implementation. The pattern is
widely seen in other HAL families as well.

* i2c: Make sure the I2C unit is disabled so we can configure it

* i2c: Support I2C transmission over MAX_NBYTE_SIZE

* i2c: Implement Rx for I2C read

* i2c: Populate docstrings

Author: keisrk