Event source independence

The core actor runtime crate of Stakker has no dependency on any event system. An interface to mio is provided as the stakker_mio crate, but it should be possible to integrate Stakker into any event system, even ones written in other languages.

All that Stakker requires is that the external event system provides it with one timeout (to wake it when the next Stakker timer expires), and that it delivers events to it as calls pushed onto the Stakker queue.