Fail-fast processors

[sylvainhalle]

Some processors (mainly of the ltl package) always return the same event once they reach some state. This is the case of:

• All Boolean (not Troolean) processors once they reach a definite (true/false) value, including quantifiers
• Any Moore machine that reaches a sink state

This fact should be detectable, to allow their instances to be cleaned up from memory (simply recalling their last value whenever needed). This would remove the need for an explicit cleanup function in the StateSlicer.