Simplified POSIX disambiguation by reconstructing capture hierarchy.

Simplified POSIX disambiguation by reconstructing capture hierarchy.

POSIX treats captured and non-captured subexpressions on equal terms.
However, non-captured subexpressions have no tags to guard them.
Previously we used default tags to infer ambiguous paths that correspond
to the missing captures: if one path had default tag and the other did
not, then desicion which path is better was made according to the leftmost
strategy. This algorithm works because in POSIX expressions without
captures have the property that leftmost path is the best path (for
example, 'a?' is greedy, it means 'a or epsilon').

However, this algorithm has one downside: because we may need leftmost
comparison, we have to impose leftmost order on NFA substates of each
DFA state, as well as maximize and orbit order for tags. This prevents
us from mapping perfectly mappable DFA states and we end up with a larger
DFA (which is sometimes folded back to smaller DFA, but not always).

Also, default-leftmost algorithm is more complex than inserting missing
hierarchy pieces: proving that it works is non-trivial.