asyncio_micro: Support just plain "yield" for cooperative control yield.
asyncio_micro: Clean up logging.
asyncio_micro: Move handle() method to SysCall base class.
asyncio_micro: Optimize size of SysCall objects.

We have to have type header for any object, so use type to "store"
information about syscall type (my initial idea was to have single
syscall class and dispatch on its attribute, that would save memory
on having bunch of classes, but would increase size of each syscall
object).