Regular constraint improvments

Alexandre Papadopoulos suggests a few modifications to PropRegular in order to improve its overall behavior.
1. When the automaton is based on large alphabet, the graph initialization could be improved by switching the two for-loops, l.217-224
2. Sometimes, one would like to define a large alphabet, but the FiniteAutomaton forbids to use some specific characters (which are '{', '}', '(', ')', '"', '<', '>', '[', ']'). Not only, there is no exception when one of those are declared in the alphabet, but also it may provide wrong solutions.