Member predicate Make::CharacterClass::getARelevantChar

Gets a character that is relevant for intersection-tests involving this character class.

Specifically, this is any of the characters mentioned explicitly in the character class, offset by one if it is inverted. For character class escapes, the result is as if the class had been written out as a series of intervals.

This set is large enough to ensure that for any two intersecting character classes, one contains a relevant character from the other.