Add xbt_assert() for state handle overflow

Add xbt_assert() for state handle overflow

With the StateManager, the current implementation
vends out integer handles that are referred to by
each UnfoldingEvent to map it to the appropriate
State.

A better API, however, may be to have a each event
manage its own state. We should verify first that
each UnfoldingEvent object corresponds to a unique
State object, but this may be the best option