Implement I2C

[bjoernQ]

This implements I2C functionality.

• implements Read and Write traits since I don't see how / if the BL602 i2c peripheral can support the contract of Transactional
• supports 7 bit addressing

I tested this with an SSD1306 (just set all pixels on and the display on / off in a loop) and communicating with an BMP180 sensor. Worked for me and looks fine when inspecting the communication with a logic analyzer.

[9names]

Code looks nice.
I gave this a trivial test, looks like it's working correctly for writing at least. I should set up a simple slave for this to poll...

It would be useful to have a way to disable internal pull-ups on the pins. It's pretty common to want external pull-ups with i2c, and you don't want the internal ones enabled in that case.
We can make that change later - feel free to resolve this by opening an issue

[bjoernQ]

Great that you already had a first look

I created #20 to track the issue about always enabled internal pull-ups so I can tackle that afterwards

[9names]

LGTM

[luojia65]

LGTM