Add I2C slave.

[matoushybl]

This adds an I2C slave implementation for I2C1 that I developed for use in a personal project. This is a very much WIP as no support for I2C2 is available and the code is not compliant with anything in embedded-hal.

It'd be nice if this could be discussed here what can be done to make this code mergeable as there are almost none examples of implementing I2C slave for STM32 and other controllers.

[therealprof]

Hi there 👋.

Thanks for this PR. I think with 3 more changes we can merge this:

• Address the commented out code, including potentially adding some more commentary
• Add a CHANGELOG entry
• Add example code so people can directly try the slave functionality