Support asynchronous I2C operations about fast-arduino-lib

[jfpoilpret]

Ideally it should be possible to implement support for a device through only one device class that could work in both sync and async mode. But it is not clear at all how that could be done.
In the worst case, we would have to provide 2 classes per device (but they should be located in the same source file, then).

[jfpoilpret]

Main idea to support asynchronous I2C API is to first introduce the concept of futures (aka promises) in FastArduino.

[jfpoilpret]

Unfortunately, after several trials, asynchronous I2C mode will be reserved to ATmega family and will not work on ATtiny (USI is not good enough to work as master in a proper async way).
On the bright side, after several proofs of concept, I start to get very positive results and support should be ready soon (end of June hopefully).

The latest PoC already allows:

• target-agnostic I2C device implementation (asynchronous mode is simulated for ATtiny MCU)
• combining sync and async API in one I2C device class (sync API calls async API and awaits)

Next steps will consist in:

• fine-tuning PoC code
• rework all I2C devices to use new async API
• add API doc to new I2C Handler
• update tutorial on implementing new I2C devices
• remove old API
• merge all to master branch

[jfpoilpret]

Quick status (on specific Git branch, not on master):

• PoC code for async I2C finalized
• all I2C devices have been ported to new async I2C library, and tested on UNO

However, I have an issue with generated code size (partly due to Async I2C implementation itself, and also Future implementation). Of course, using asynchronous I2C, you shall accept the extra cost for it.

But this extra cost is also present for I2C devices sync API (which uses async API). I would like to remove this extra cost for programs that exclusively use sync API. But this is not obvious unfortunately. I need to study possible options for that.

What all this means is that, unfortunately, I will have to postpone milestone 1.6 at least one month.

[jfpoilpret]

Implemented on master branch at commit 8e39e6d
Will be released in v1.6