i2c permits forgetting to call set_slave_address

[ijackson]

The rppal gpio functions use the Rust type system to stop you forgetting to set the pin mode, etc. This kind of thing is a good idea.

i2c should ideally have something similar, where you can't forget to call set_slave_address. (AFAICT none of the other setters are mandatory.) Unfortunately I think this would be a breaking change.