i2c: Reimplement with new HAL API design

[Rahix]

Pull off the same design that the USART module is using now in the I2C HAL as well. The key points are:

• avr-hal-generic defines the user facing API type (e.g. I2c<> or Usart<>). This type implements embedded-hal traits and provides all API which we want to expose.
• There is a trait defining the lower-level API, called I2cOps (or UsartOps). This trait is "consumed" by the I2c<> type to access the peripheral.
  • The methods of this trait have the prefix raw_ and are not meant to be used directly by the user.
• A macro exists which implements this trait for a peripheral. E.g. impl_i2c_twi!{} in this case.
• Because the HAL crate neither "owns" the I2cOps trait nor the peripheral type (it comes from avr-device), we run into the "orphan rule". To "fix" this, the I2cOps trait and I2c<> type have a type-parameter called H which is substituted by a local type from e.g. atmega-hal.

Cc: @explicite