I2C: Prefer compile-time checks over runtime checks where possible, prefer a fallible API over panics.

[bugadani]

• There are a number of unwraps. In set_interrupt_handler it's a common pattern and the function is infallible, but configure_clock should instead return errors.
• Some panic!s listed (just to revisit if we can improve here): https://github.com/search?q=repo%3Aesp-rs%2Fesp-hal++panic%21+path%3Aesp-hal%2Fsrc%2Fi2c%2Fmaster%2Fmod.rs&type=code