State machine execution logic

[psoetens]

This PR modifies the state machine execution logic in a non-backwards compatible way, but does introduce clear semantics of what is done when and in which step of the TaskContext.

Open questions:

• Shouldn't events be enabled already in entry{} ?
  • today they are not, and since we sleep after entry{} there's no way to intercept an event that came in during entry{} and respond to that. So the thing causing the event needs to move into run{}, but then a default transition will be taken if present because the event did not have time yet to be processed if it's send().
• Should the user have the choice where the SM goes to sleep ?
  • between exit{} and entry{}
  • between entry{} and run{}
  • between run{} and transitions {}