CO: Simplify xpcc::co::State enum.
CO: Move CoState into xpcc::co namespace.
CO: ALWAYS_INLINE -> inline. The compiler knows best.
CO: Reduce local continuations size to uint8_t by using __COUNTER__ magic.
CO: Add CO_RETURN_CALL(coroutine) macro for completion.
Update drivers to use it.
CO: Update documentation a bit.
UI: Move interpolation code into its own class.
Adapt Animation class for changes.
I2C: Fix Operation enum forwarding.
CO: Fix coroutine macros.
[UI]: Update Led tables for gamma correction to an exponent of 2.2.
[UI]: xpcc::ui::LinearInterpolation -> xpcc::ui::FastRamp.
Add contructor and update documentation.
[UI]: All parameter tuples of (time, brightness) -> (brightness, time).
[UI]: Rename xpcc::ui::Led.brightnessAnimation -> Led.animation.
[UI]: Remove virtual function setValue(uint8_t) from
xpcc::ui::Led in favor of a standard function pointer.

Now subclassing the Led class, just to use custom update code
is not required anymore.

This also makes the use of xpcc::ui::Animation a little more,
flexible since it can now directly call the update code,
instead of only returning `true` from its update() method.
[CO]: Move CoState back into the class to make it private.
CO: Update documentation of Coroutines.
I2C: Beautify.
UI: Beautify KeyFrame animation documentation a little.
Driver: Clock: Remove unnecessary atomic lock for ARM CPUs.
The bus has atomic access to 32bit, the counter is only 16bit.
Clean-up AVR examples a little more.