I2C slave mode

[xek]

Slave I2C

implemented using the framework interrupt handler from ST (AN3281). The master I2C code in sduino is untouched.

slave_receiver and slave_sender examples have been updated with the sduino C-style function definitions

added an example, showing STM8 as a I2C serial eeprom. The example demonstrates the use of both onReceive() and onRequest() callbacks

This is a PR created from the changes from https://github.com/stefaandesmet2003/sduino in case someone finds it useful.

Fixes #79.

• I2C slave implementation
• Added slave_eeprom example

[mcer12]

This will come handy, thanks!